I wouldnt just make the jump to Wii because they appeal to younger audiences. I would seriously look into the 360 before buying a Wii. Although the games seem more appealing when compared to the others they like, a lot more are available on the 360. With better games and hardware i would suggest one of them.
I own both of the consoles and although I do like both, I definitely do prefer my 360 and own a lot more games on it.
But its down to personal preferences.
I would definitely recommend a Wii if you are likely to regularly play multiplayer, but i would recommend a 360 if you are likely to do as much single player if not more than multiplayer.
Would recommend Mario Galaxy if you make the choice of Wii as well as Super Smash Bros. Both fantastic games in my eyes (Mario Galaxy is by far my favorite game on the console)