The PS3 will be a more powerful system. That said, the 3 next-gens will all be insanely powerful machines, and the games that come out in the first year or 2 will all be on par with each other. Down the road, when programmers get to the point where they can get closer to the full potential of the systems, the PS3 will be able to do a lot more, but how much better is it going to be. You'd think there would be a certain point where things will be so realistic that slight improvements wont really be as drastic.
I'm getting a PS3 though. The X-Box's main draw was the Halo series(which I do not like, FPS games bore me). PS has always been a better sports game console in my opinion, and that is what I play 90% of the time. That and the GTA and Metal Gear games when they come out. I like RPG's too, but I just play those on my computer.