Well.. It depends. First, what kind of games do you want to play and would you want to play online or not? Shooters? RPG? Racing? The graphical quality of them is not so different from each other for games.
Aside from that, do you buy Blu-Ray discs? If so, the PS3 can play them, the X360 can't. Both can play DVDs, PS3 has better audio support, but nothing noticeable unless you have a 500+ dollar sound equipment.
Bottom line, if you want some better advice, I'd need a bit more information.