Now do I like the idea of the established set-up from the first movie remaining as it is, and things playing out the way you said? Yea sure, why not, could've been really cool.

But in that case, I'd prefer they'd have no "twist" shoehorned into the story at all - let Vader just tempt Luke with ruling the Galaxy or whatever, and then continue spinning the plot.

To the question "which" twist I find better, I can definitely say the one that was in the movie. Surely, Obi-Wan being the bad guy all along would've been more SHOCKING, but think about it - it would be a more disturbing case of reframing and retconning than even the worst "hot pants Leia's your sister" boner killer.

The warm-hearted, fatherly, good mentor from the first movie... that keeps hovering around you as a helpful guide ghost... is the VILLAIN? Shocking, but so shocking that it turns right around to cheap.

Vader Luke's father? Pretty shocking. But exactly as much as it needs to be.

Just for the record, I'm not saying you can't have a story where the hero's mentor is an evilhead, or that it isn't an archetypal trope, as well. Or that it doesn't make sense.

Spiderman 1-2 did it, but went the easy route of "alter egos". Batman Begins did it, but the hero's relationship with Qui-Gon Jinn was only established in like a short montage prologue, and the guy once again acted with such understatement, and the ninja clan revealed as villaneous so early in the movie, that it lost any effectiveness whatsoever. Matrix 3 could've done it with the Oracle, but didn't dare apparently.

In this csae, if you just took the movies as they are, and "swapped the twists" in an otherwise almost identical scene, the one with Obi-Wan would certainly, certainly be worse.