Its all a matter of choice. And also what you want out of a games console. At the end of the day, both consoles play great games, really well, have a couple of neat little extras such as online gaming and so on.
Probably the best way to decide is look at the games available for each console and decide based on that. Your son may have a reason for going xbox rather than ps3 (ie most of his mates may have 360 so he can play onnline against them etc, swap games etc)
Its like arguing the case of what mobile phones better. A nokia N95 can do a whyole range of things, but at the end of the day its makes calls and sends texts, same as the phone thats a tenner is asda. It does what its meant to do, lol
(PS3 all the way tho
)