Forget what everyone else said above (just kidding) and take my advice -- I think you should go with the Wii U. As mentioned above, you can play your Wii games on the Wii U. I know you stated that you only want that one Wii game, but you actually might find yourself liking the Wii U more than you expected. In that case, you could then purchase other Wii U games. You would have possession of the latest Nintendo product and wouldn't have to worry about updating for the time being.
I know the Wii U is more expensive than the Wii U but you should consider how it would be in the long-run. You have your Wii capabilities on the Wii U and the option to get other games if you fall in love with the system. Don't sit in regret, wishing you could be playing all the great games out now.
Sincerely,
ChiefKeef
P.S. I'm always right.