Buying Wii or Wii U?

No...

Yes. We are not there yet, so one generation back compatibility is not bad and is almost always included, but in a decade or so games will not even be sold in physical copies to begin with. It is not like backwards compatibility is a historical standard either, we have only had it for 2 generations of consoles, more or less.
And in most instances, if you have the game, you have the console.