Your best bet would be to go to Wikipedia or Metacritic or something, and just find out about a bunch of games on both consoles. That's pretty much the make-or-break factor when buying an item specifically for the purposes of playing games
For my money, I've been extremely satisfied with my 360 purchase since day one, and it's got great games like Dead Rising, Gears of War, Halo 3, Earth Defense Force 2017, GTA IV, BioShock, Oblivion, Call of Duty 4 - you get the picture. It's a pretty action-oriented system, though, something you might want to keep in mind.