TROLLLLLLLLLLLLLLL!!!!!!!!!!!!!
Get both. And Steam.
Or if you really must decide then look at the games on each console. A console should be about the GAMES. And in regards to games it an only be your choice. Whatever console has the more preferable games, then get that console.
Here's my collection (not including cross-platform games, unless there is significant reason to take one over the other), maybe you can make a judgement from it.
Xbox 360:
BioShock (generally runs better)
Crackdown
Eat Lead (meh)
Gears
Gears 2
Halo 3, ODST, Reach, Wars
Mass Effect
Mass Effect 2 (this is different next January, the PS3 version will be out with all DLC included. However as yet ME1 will not be available and therfore you will miss out on making decisions in ME1 that carry over to ME2.)
Metro 2033
Splinter Cell: Conviction
Universe At War
PS3:
FFXIII (runs better, only one disc as opposed to 3)
God of War Collection
God of War III
Heavenly Sword
InFamous
Killzone 2
LittleBigPlanet
MGS4
MotorStorm
MotorStorm 2
Ratchet & Clank, Tools of Destruction, Quest for Booty, Crack in Time
Resistance: Fall of Man
Resistance 2
Uncharted
Uncharted 2
Personally, I prefer my PS3. Xbox 360 runs some earlier cross-platform games better though. But more recently there's no difference.
EDIT:
Steam:
Haha JOKES. I'm not gonna list all 200+ Steam games here... But I don't even have that great a computer and it still runs stuff fine. And it's a laptop. It's a 512Mb 8600GS M with a 2.0Ghz dual core and I was playing the MOH beta so whatever tickles your pickle really.