Are you going to be using it mainly as a game system and nothing else? The 360 is gonna be better in that regard, especially online (except the fact that you have to pay or it). But if you want a better "media center", or even just a top notch blu-ray player, then I choose the PS3.
That's mainly why I got a PS3, because it was more than just a game system...and with a 500 gig hard drive, can't really go wrong 😀...and also was the fact that my dad surprised me one day after school with it sitting in my living room haha.