If you're on a budget, I'd get Mario Kart. Its really a fun game and some of the new courses are really great.

WiiFit is going to be pretty cool but its $90. (Assuming you're in the US.. I think its about the same price in the UK.)

Short Term: Wii Fit
Long Term: Mario Kart Wii

Go for Wii Fit if it's in Stock as you can get MK Wii, but Wii Fit is sure to have a shortage, also MK Wii is more likely to come down in price faster then Wii Fit.
I waited a week and got MK Wii for £32 instead of £40

I vote : Wii Fit.

I haven't played either, but I'd get Wii Fit. Mario Kart is the safer option because it's more traditional and not much different from its predecesors. Wii Fit on the other hand is something quite novel.

Mario Kart!

Then we can play. :)

I decided on Mariokartwii. THEY DIDNT HAVE IT!!!!! ITS SOLD OUT!!!!

THERE WERE MORE COPIES OF WIIFIT THAN I CAN COUNT BUT NO MARIOKART!!!

I didn't buy Wiifit, upon reflection I've decided it would bore me quickly. I'll get Mario kart.

Wise choice. Mario Kart is amazing!

I own MK and Wii Fit here in Belgium. I love them. Both have its way.
For MK, you have to have in your Wii the WFC for Online matches, competitions, time trials, ghost sending, battles,..
Don't listen the guys which say MK graphics suck.
Concerning WiiFit, for me, it's like a brand new fresh system. You don't have to be big to buy it. There are some muscle and yoga exercices which are really challenging too and with 10 days, I saw (a little) changing on me.
The advantage of WiiFit, it's that more and more games will use the balance board in the future (10 games in preparation like We Ski,available next week, a Wario Ware, Rayman Rabbids, an EA game,..)

Now, it's all about you.

You should get Wii Fit because you're too fat, and you know it. Get your lazy a$$ off the couch nao.

I got Wii Fit because I'm too fat.

Nice one lamer.

WiiFit is awesome - for what it is. And that is a "Fitness" game.

It can't be compared to Mario Kart, any more than say watching a good movie can be compared with going on a long walk with friends. They are completely different.

I would recommend MK more to a "gamer" though, as it is a GAME.