I have a Wii, xbox and a PS3
The wii is great a social gaming machine, but dead boring on your own.
The PS3 is top notch technology and comes with a blueray disc drive which would look awesome on a 42" HDTV. The things I hate about the PS3 are the crap games and the flimsy controller. The only (half) decent games imo are GTA4, GT Prologue and my current favourite is metal Gear Solid 4 which is awesome. i only have my PS3 just incase something decent and excusive comes out on it.
The Xbox is a quality machine. They can be picked up for a bargain now and the controller is quality imo. Also there is a massive catologue of great games available and more importantly there is a massive xbox online community which is a humongous part of gaming today.
Someone mentioned that, what his friends have is important, that imo can't be understated. No point your kid having an all singing all dancing PS3 if all his friends have Xbox's, you don't want him to get bullied do you? When your a kid, no one likes a show off, its all about fitting in