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.