Buy whatever you can afford.
And don't listen to fanboys.
Also note that if you plan to play online, you have to pay on Xbox360 but not on PS3. That's probably the biggest difference between them.
XBOX: 1st person shooters
PS3: Anything else
The Xbox community is pretty immature, but I went with it to play with my friends. The fee for online isn't much either, just pay once a year for $50. So basically, pick the one that your friends have, and the kind of games you like.