Wii because it is more active. They can play sports games to get them moving, or can play a normal game like epic Mickey, sonic, Mario, cooking mama, etc. my second choice for a kid that age would be a play station manly because Xbox is a lot more complex.
Wii has more age appropriate games (in my opinion) for a six year old. However if you are after a system that will last a while (I think the wii is dated) then go with the Xbox with kinect. The kinect offers good games for younger kids. And as he gets older and can play the "big boy games" Xbox also will have those to offer.