Actually, I'm kinda torn on GTA IV. I hate the way Microsoft is going about this, paying money to make one version inferior. It bugs the hell out of me and I kinda want to buy the PS3 version just to spite them. It just stinks too much of their dirty tactics when it comes to Windows and I don't like seeing that from the Xbox division.
Additionally, both of my roommates own PS3s but no 360s. If I want to play the MP aspect of the game with them, I'd have to get the PS3 version... at the same time, it seems likely to me that if one version performs better than the other, then it'll be the 360 version.
No clue which version I'll actually get. If performance really is equal across the two versions, then I'll just get the PS3 version to spite Microsoft for the way they've been behaving this generation. If the 360 version performs noticeably better (like in Assassin's Creed), then I'll suck it up and buy that version.