Well its hard to really compare because it literally would be an entirely different series, with entirely different plots, characters, environments and overall tone. Because of that, it is actually a very good question.

Dark Horse tried to answer this with their "Infinities" line of comics, wherein they present alternate versions of the OT but with knowledge of the PT--they turn out quite different from the films. These are, however, mosly "what if?" type of stories and not true PT-centric OT remakes.

Had the PT been genuinely been made initially, the PT itself would first of all be quite different, because it wouldn't have to fit around the OT. It would, i think, be more coherant as a whole because of this. The first two episodes would be quite similar, maybe even exactly similar with only a few deviations, as Lucas mostly made these up from scratch. This gives you a good indication of where the series would have headed and what the OT might be like. Lucas has said that he had to basically re-write all of Episode III once he had completed the outline in august of 2002 because he realised it had a structure that did not align or bridge into Episode IV smoothly--it was a natural continuation of the PT plot which left in Episode II, and probably put more focus of things that were sort of left by the wayside in the final version of Episode III, such as all the plotting surrounding the Jedi, the Force and the prophecy, Sifo-Dias, the revenge of Boba Fett, and the turn into Vader occured later on in the film as far as i can tell. The droids would not be in it. And Leia would not be Luke's sister.

There are two ways the arc of the OT plot could go: one with the two death stars, in Episodes IV and VI, and one with only one death star in Episode IV. In any case the film would focus more on Luke and Obi Wan, instead of an ensemble character piece. Tarkin of course would not be in the film at all, and instead Vader would be the villain, and all the bad guy scenes would be set on the Imperial Palace on Coruscant. Princess Leia would be captured as a traitor to the Empire in the opening scene and taken as hostage in a similar sequence to the film, with R2 and 3P0 escaping to warn other rebel members to aid her. Maybe she could have knowledge that the Empire is in the midst of constructing a doomsday device somewhere and was en route to secure Obi Wan as in the final film. Luke would be introduced in a thrilling Skyhopper racing sequence, such as in the radio drama. Obi Wan's story of the old republic and Anakin of course would be greatly different, but maybe he would still hide the truth about Vader's identity, and this would be the grave secret he could keep in guilt for the film. The rest of the film would feature Luke and Obi Wan recruiting Han to rescue the princess from a prison complex on Coruscant--only Han would be a CG alien, perhaps the green fish-mutant he was in the 1974 rough draft. There would of course be a scene of the Emperor dissolving the Imperial senate. If Lucas went with having a Death Star in the film it would be closer to the final version, although Coruscant would still be shown, perhaps with a scene between him and the Emperor in there somewhere. A scene of Vader sensing something amiss due to Luke's presence would also be in there somewhere i am sure. There might even be a section where they first travel to another rebel-friendly planet and recruit a bunch of alien rebels, who then accompany them as a large assault squad to retrieve the princess, much like the 1973 treatment, resulting in a much bigger battle but one that probably is much less humanised and entertaining character-wise. The film would be more dramatic and dark, and Vader's rematch with Obi Wan would be a much bigger focal point with much more gravity and flash i am sure.
After the princess is rescued/Death Star is destroyed, there would be a scene with Vader travelling back to Coruscant: he arrives in the gloomy Imperial Palace, rain pouring down and kneels before the Emperors throne, telling his master of the defeat. The Emperor throws Vader around and chokes him as punishment, telling him that he must not fail him again. Vader weakly apologises and the Emperor orders him away, and he storms away in shame, throwing aside Imperial officers. He stands waiting at a balcony some time later when an officer comes to deliver him news: they have learned that the identity of the one who rescued the princess/destroyed the death star is named Luke Skywalker. Vader's breathing silences for a tense moment and he stares back before his breathing returns to normal. He quietly thanks him and ordering him away. The scene would end with him contemplating the news as he stares into the rainy coruscant skyline. Fade back to the Yavin medal ceremony. Stuff like this no doubt would be peppered throughout the series to give insight into Vader's character that is needed due to the PT but lacking in the actual OT.
The second film could basically go any way needed, but it could also stay pretty much as is, though given the possibilities available to Lucas i am sure that he would come up with a much bigger and alien-central plot that focuses much less on the core character intimacies than the real ESB does. Leia and Han's relationship wouldn't be such a big part, and because of that Lucas might not even split up the character group as he did, although i don't know how that would effect Luke's training. Yoda and Obi Wan would be bigger characters, perhaps discussing a plan more coherant and clear with regards to Luke's purpose. The equivalent to Lando would be introduced in an action scene on Coruscant, with him and Bothan spies (CGI aliens naturally) undergoing a complicated mission to retrieve the sensitive Death Star info--likely Han, Leia and Chewie could partake in this as well. The rebels would be much better developed in terms of politics, with it revealed that much of the general populace was being swayed to side of the rebels due to the Empire's growing iron fist, and many key leaders would be integrated amongst the politicians and general citizens the way Leia was in Episode IV. I doubt that Han's cliffhanger would be included--probably the film would end with the Rebels preparing to have an all-or-nothing final assault, maybe with the cliffhanger being that the Empire's planet-destroying doomsday device is completed.
Episode VI would probably be the most different. Lucas has said that he made the rescue of Han such a big part mainly because the character had become so popular, but i doubt that he would have been as popular without Ford or such a big emphasis on him. If Lucas chose to go the single-Death Star route then this would probably have most of the scenes smilar to that of the film, except with the Emperor remaing on coruscant. The film could perhaps open with the Emperors visit as in the final film, which introduces Vader's relationship with him and sets up the plot, but have him not stay. Like the initial 1981 rough draft, the Death Star would remain by orbitting Coruscant, with Endor a sanctuary moon being harvested for resources. Luke would visit Yoda and build a new lightsaber before Yoda dies and Obi Wan fills Luke in on the real history as per the film. The film would progress from here, except the Ewoks would be different--they might be wookies, as per Lucas' original conception but then this would necessitate that Chewie be another type of alien, preferrably of the CG kind, which is more in keeping with Lucas' tastes of CGI-alien-sidekicks. The Endor battle would of course be more massive and epic, but a smaller focal point. Han, Leia and the CGI-alien-Lando-equivalent would partake in the Death Star battle after destroying the shield generator and Luke surrenders to the Imperial troops, who take him to Vader. A similar temptation scene as in the film would follow, and Vader would then take Luke to Coruscant.
Perhaps there might be a scene wherein Vader shows Luke the desecrated old Jedi temple before he takes him to the Emperor, with the temple now converted into a Sith shrine. Obi Wan could then appear and tempt Vader to turn good, as in the 1981 rough draft as well. Vader would then take Luke to meet the Emperor, who sits on his throne atop a lake of lava in the bowels of the planet as in the 1981 rough draft as well. The Emperor would be competing with Vader for Luke, with each plotting to kill the other, as in the 1981 rough draft, and this would be a simialrly central issue, with Moff Jerjerod being a central villain as he was in that draft. Perhaps it could be structured simialr to that, where Jerjerrod has Luke delivered to the Emperor behind Vader's back--Vader becomes enraged and kills Jerjerrod and storms the Palace to save Luke from the Emperor, killing Imperial guards who stand in his way. He finally confronts the Emeperor, who begins choking Vader until Vader submits. This whole lengthy section was one of the most powerful parts of the 1981 draft and it would probably be reprised, building much pathos in Vader. The temptation scenes with Luke would probably play out similar to the film, although the Emperor's manipulation would probably not be as blatant and illogical as it is considering how much work it took to sway Anakin in the first half of the series. This aspect could probably be integrated into the films better. The Death Star could be destroyed much quicker where the battle then goes to Imperial City on Coruscant. If there is only one death star in the series and this not even an aspect of ROTJ, then the shield generator plot would be to deactivate the shield that protects the Imperial planet so that the ships could then descend in an assault. The Emperors navy would of course be waiting and a simialr showdown would appear, which then takes to the inner atmosphere, maybe reminiscent of Episode III before descending into an all-out destuction of the Emperors palace. The Rebels integrated in the planet itself would then take the assault as sign of galaxy-wide rebellion and rise up with the citizens, resulting in a true planet-side civil war. Meanwhile, Luke gives into his anger and strikes down Vader in the Emperor's cavern, as the place crumbles apart due to the battle above, perhaps a nice symbolism of Luke's destructive power. He refuses and the Emperor starts to kill him, but Vader has his redemption and throws the Emperor into the lava. Luke and Vader then have their reconsilliation as in the film, perhaps in the palace docking bay above. Finally Luke brings his father's body into a ship and escapes as the palace falls apart and explodes. On the planet outside stormtroopers are overthrown as the planet topples its oppressors and Leia is elected as ruler, perhaps with Han as her king in a sort of humorous reversal of power roles. The final coda would have Luke rebuilding the Jedi temple and vowing to start anew, as the ghosts of Yoda, Obi Wan and his father look on proudly.

Or something. I'm sure the films would be startlingly different from anything i can speculate but i think this is at least a somewhat accurate estimation.

Cable-X1 said:

I think the PT made in the 80s would have rocked the hell out of us. One of the downfalls of the PT was that Lucas waiting too damn long. He shouldn't have let us catch a breath after Jedi. Maybe given it a year or two and then launched into it. That would have been much better.

So true about accepting the inferior ROTJ...I'll take the Ewoks over Jar Jar any day.

 

 It's true.  Without decades of "catching our breath" and "hearing all of the various plans and rumours as to how things would shake out" and "using our own imaginations which turned out to be better than George's" we would have all like the PT much better.  It probably would have been made better, but who can say?

 

The other factor is that there wouldn't have been as much extensive EU before the prequels either.  I know that Lucasfilm said the PT timeframe was off limits to the EU authors, but that didn't stop The Thrawn Trilogy, Dark Empire, Children of the Jedi, etc... from casting long shadows back on galactic history, especially in regards to the Jedi and the Clone Wars.  It still makes me wonder what some of their other ideas were for the Clone Wars.  Probably superior to the ones we got., or would have gotten in the late 80s.  So maybe it's not all bad that the PT didn't come out right away.

I often poder that question myself. Yes, sometimes I think "Why did he not make them first?" but after all, I don't hate the way things are. It would be easier if A New Hope was Episode I. We didn't really need to see how that shit went down. We bounced like a dog staring at a beggin strip in his owners hand. The whole problem with Star Wars is, we get things explained to us. Blader Runner was the weirdest movie when I first saw it. There were many questions I asked myself. I watched it a few times then understood it in the end. Star Wars would have been so much better if things weren't explained. It isn't like it is a movie you have to pay attention to. You know what is going on in A New Hope. Then you get to the prequels and you are spoonfed every single little detail. It takes the fantasy out of it when someone has to explain it. I would rather remain ignorant to those little details then have them shoved in my face like "this is who/what/where it is" So the films were all quite good. Just episodes I-III had the 'Fantasy' aspect removed. Yes they were epic but you knew everything. We are not all that dumb, though don't get me wrong I had a girlfriend that was. It is just a matter of paying attention or not. And if you are at the theater you are forced to pay attention. My eyes are wide open George Lucas. I need not know who he/her is or what this/that is. Those little things distract from the main characters. All we need to know is

1. Who the Hero/Leading Characters are

2. Who the bad guys are.

3. Why they are fighting

Presto that is it. It goes back to the days of the stage. When all we had was human beings and a few props. We don't need anything else to be entertained. Not saying avoid the special effects. Just focus on characters who are important. Other characters can just stay in the background. Instead every character in the prequels seem to have a significant story that is explained in the film. Boba Fett would have been way cooler if he was just that guy who popped up in Empire Strikes Back who said 4 lines. Instead we are given a young child who looses his daddy and is sad. It is so much better to just wonder who that guy is than actually know. So the prequels werent bad stories. They werent executed as written.