It depends on what you want to be playing, really. As you see, the only big PS3-exclusive you're interested in is MGS4. A lot of the time games are on both platforms, but have differences (And not just technical ones.) When it comes to online stuff, the 360 is frankly excellent most of the time. Friends, DLC, Arcade, it's pretty fracking fantastic. I can't speak about the PS3 properly as I don't own one, but that system has its own strengths.

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.