I'm going to assume you ONLY have a Wii, since you haven't mentioned other systems.
I would say wait for the Mario or Zelda game if you love those, the WiiSports stuff is generally for when you've got people to play with. At least that's how I've always played it. I've only ever played when I could play with a friend or family member. But you might be different from me and love playing the original WiiSports alone. XD
The WiiFit thing could be fun, but that really goes off your personal opinion, I'd say...I haven't gotten it, but that's simply because I have little time to do that sort of thing. :>
But I, personally, would wait for Mario Galaxy 2 or Legend of Zelda. Those games take quite awhile to lose their playability and I'd say you probably would get bored with the Resort mini-games after awhile, especially if that's one of few things you have to play. But like I said, those are mainly my opinions. :> Hopefully you enjoy whatever choice you do make.