I have both, only ever play XBox, hardly know anyone with a PS3, and those people i do know bought it more for its bluray capabilities and never play online.
Xbox live is £30 per annum, PSN is free but you definately get what you pay for. Live is intuitive PSN is not IMO.
The main decision maker for me was the controller, PS3 controller is very dated and hurts my hands, Xbox controller has a more natural fit.
older Xboxes suffer from RROD and disk drive failures, both are very easy to fix and cost less than £20 if you do it your self.
Fat PS3's suffer from multiple issues and are more difficult and expensive to diagnose.
In my opinion, see what the majority of your mates have got and get that, you wont be loosing out in terms of graphics or playability which ever console you buy
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