Buying a console is actually a significant downgrade from PC Gaming.
But I say a Wii.
He's got a PC, so he's got the best gaming platform.
So, get the best party platform--the Wii.
I agree with this. He's already got a great single player gaming system. His pc will run anything that comes out for both Ps3 and Xbox.
It really depends on what he'll be using it for. If he likes to entertain people, or just play funny/entertaining games, then the Wii it is.
If he wants another hard core gaming system, then one of the other two. If he decides this, then he'll just have to weigh the pros and cons between the two.