My Son is also 11. He lives on the XBox. All of his friends have XBox, they sorta dislike the PS3 online play. I also enjoy playing the XBox myself, you might get addicted too if you aren't careful. haha
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Im a PS3 guy and I would suggest getting him one because the controller is what he is used to, but if 99% of his friends have X-box 360 which is probably the case I'd get him the Xbox 360 so he can share games,play online etc. or else you may find the system sitting as he goes ahead and plays xbox at his friends house.
Go with the PS3. The PS3 allows you free online play, where the X-Box will charge you to play. Plus, X-Box is also known for the red ring of death. Personally, I spent way too much time playing GT5 and MW3 on PS3.