I think the PS3 is more for Singleplayer sort of games, especially the button layout (with both sticks at bottom center of controller), and a very friendly online community for when you do go online. Its free but unfortunately also does come with some downfalls such as security(Which of course is better now that they know whats possible), and also Licenses usually mean PS3 gets DLC and such AFTER Xbox.
Xbox is more for the competitive person. the community is usually full of (if I say so myself) assholes who are all looking for more competition. The controller itself is more for shooting. If you like to be the best in the lobby and always win your games, get an xbox for sure. The bad thing is Micro$oft is very costly, and also the quality of things (such as headsets and controllers) in my experiences, and also stories form those I know, is not so great either. One specific example is the headsets...