I think Wii Fit U will dissapoint honestly. It's an old idea by now and the market has been flooded with fitness games, a soccer mom is not going to pay $300+ play a few more Wii Fit mini-games.
I think it will perform a lot like Nintendogs from DS to 3DS.
Nintendogs DS: 23.6 million (approx)
Nintendogs + cats: 2.7 million (approx)