As an owner of both, I would say buy an XBox 360. To me, it has a great list of games and awesome Online support. PS3 is very nice for the first party titles that I've wanted to play, but overall I find myself playing the 360 a lot more.
Quick list: XBox 360: Gears of War 1/2, Mass Effect, Fable 2, Left 4 Dead, Halo Wars, Command & Conquer 3, Braid, GTA4: Lost and the Damned (another DLC coming).
PlayStation 3: LittleBigPlanet, Killzone 2, Metal Gear Solid 4, God of War 3, Resistance, Fat Princess...
I will say though that the XBox 360 does have a better 3rd party game support. They tend to have games that run slightly better on the 360 and more DLC for the 3rd party games.
It's really up to you though, but that is my $.02.
XBOX LIVE: macr0planet
PSN: MacroPlanet_DSGN
Steam: MacroPlanet
Modern Warfare 2 - Uncharted 2 - Assassin's Creed 2 - Mass Effect 2 - Alan Wake - Forza 3 - Splinter Cell: Conviction - God of War 3
