Ok, I am a hardcore player. The 360 and PS3 compete amongst themselves because it's what most hardcore gamers use. In fact the Wii is a whole different category of console and does not compete with the PS3 or 360. For an 11 year old, I'd recommend the Wii. Also it's great for the whole family.
Aside from that, PS3 has a few advantages, like a much more relaible hardware, and you can play online without paying extra, unlike the 360. What holds the PS3 back, is that they have about half of the games that the 360 has. But there are about 200 new games scheduled for release next year for the PS3.
I am a fan of the Playstation systems, I don't trust anything made by Microsoft and most ppl I know with the Xbox, enjoy the gameplay greatly but 4 out of 5 have had to send the console back for replacement at least once, sometimes 2 or 3 times. I just got my PS3 last week and it f'ing rocks.
But again, if it's mostly for an 11 year old, you can't go wrong with the Wii.