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.
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