That depends, xbox typically is more geared toward people who are going to be playing the big first person shooters, and getting competitive, and PS3 is more laid back, story games, whatever.
Obviously you can still play COD on ps3, but like i said, its less competitive.
If you don't have many friends that play on console that you're going to be in a party with talking to all the time, and just plan on not really talking at all, or to just the people you play with, get a ps3. Sony's single player exclusives are also pretty good.