Twisters theory is actually pretty cool :)
tarkin probably have had more governing responsibilities. but as far as hidden power and, more importantly, from a cinematic perspective vader was the main villain. Forget the force powers, the scary breathing and all the merchandising - He almost single-handedly took down the entire rebel assault on the death star (perhaps this was the first clue that anakin and vader were the same person).
lucas in his 2004 DVD commentaries saying "vader was never supposed to be powerful villain, he was a pathetic character" disgusted me beyond belief. i wasn't fooled (nor surprised) by this at all and knew right away that lucas' real reason for saying this was because he knew he botched the ressurection scene in episode III and was pre-emptively trying to spin it; knowing full well it was anticipated to be the climactic moment in the entire PT. Its no different than all his other "well, it wasn't supposed to be this, it was meant to be that way all along" talking points. lets see how many interviews prior to 1995 did lucas say "vader is a pathetic character! he's lame!! loser! limp! wimp!!". and no, tragic is NOT the same as pathetic.
sadly, apologists on TFN bought into it and started turning over rocks trying to find things in the OT to justify it. I mean, for cripes sake, he's a giant guy dressed in black and the camera shows a CLOESUP of him lifting someone off of the ground! lucas doesn't do multi-facted characters with ambiguities. When you want a pathetic character, do you really want james frickin earl jones to do the voice!?
In the end, I still liked the resurrection scene cuz...well, even lucas cant ruin vader. and when that helmet came on - that was the kiler shot. But as reports from sneak previews were pouring in about how bad this scene was, it didn't surprise me at all. and lucas again blabbing on "he's pathetic! he's pathetic!! nobody likes him!! palpatine is bummed out he has a broken down sidekick!! its supposed to be like this!! i had it planned exactly like this in 1977!!!" um, yeah sure. palpatine was so disspointed that he smiled when vader broke from his restraints.
[/rant over] (do people still say '/rant over' ?)
anyway, was still cool seeing tarkin in episode III. but the fact that he was just a cameo i think strengthens the argument that the empire ws palpatine first, vader second.