I've had both. From experience these are my opinions:
PS3:
Pros
- Free online
- Blue Ray
- Good single player games
- More reliable
Cons
- Compared to Xbox not many people online
- Not very good multiplayer games
Xbox 360:
Pros
- Safer online
- You can play just about any game and people still play online. ex Call of Duty 4.
- Many games geared toward multiplayer
- More FPS games
Cons
- Online community is shit. (But playing with friends and the mute button an solve this problem)
- (If you buy older box) Needs to be wired, or buy a wireless adapter. about $60
- More unreliable
Just my opinion. I prefer the 360 because of its massive online community. I get bored quickly with single player games. After you beat it there isn't much to do. But multiplayer is always changing and more fun imo.