We have both PS3 and Wii systems in my house. My 8 year old son plays both, but he prefers the PS3 in general due to game variety. That could also be because the PS3 is in the HT with the Axioms too!
However my daughters who are 4 and 10 pretty much only play the Wii - they are not gamers like my son is. Of course the 4 year old is happy just pretending to play - we give her a controller that's not connected and she thinks she's playing! The ruse usually lasts long enough for her to decide she wants to do something else.
So to try and help - if they really like video games? Go with XBox or PS3. Casual play or family fun? Wii is probably a better choice.
