I have an SR MBP. Maybe i should go with XP for now and wait for Vista SP1 or a time when I actually want to play a dx10 game. Basically I just want to play some oblivion and BF2.What mac do you have? IF you have a mac with a Dx10 video card, then it MAY be worth your while to go with Vista, especially once SP1 comes out.. that's supposed to speed it up a lot.
If you don't have a Dx10 video card, then I'd stick with XP since you're not going to lose much if anything I don't think, and gain a few extra FPS's since there's less overhead from the OS.