I’ve no idea what games you like, but I would say get the Wii then wait for a game you really really want on the Wii U before purchasing.
I like all the Nintendo’s own games like:
Super Mario Galaxy
Wii Sports Resort
Zelda Twilight Princess
Zelda Skyward Sword (imo the best zelda since Ocarina of Time).
But if you fancy something a little more ‘adult’, Resident Evil 4 is one of the best experiences ever on Wii.
Metroid Prime 3: Corruption
(or the trilogy if you’ve got 100s of hours to spare) is also really good.
These games sold so many at launch, that now they’re cheap as chips.
Another vote for Wii & balance board.
Oh, and on the topic of M-rated games, get a copy of House of the Dead: Overkill for cheesy grindhouse B movie-style zombie shooting. More humour than RE games, and better music.