MKW, but not by much, and I expect them both to sell way better than their predecessors.

Smash Bros. It was the best selling GameCube game ever.

Taken from Wikipedia.org:

being the best-selling game that has been released for the GameCube with six million units sold worldwide.[11]

mario kart 64: 9.87 million

smash bros 64: 5.55 million

mario kart DD: 6.60 million

smash bros melee: 6.58 million

mario kart ds: 8.81 million

a smash bros game has never outsold a mariokart game.

According to the internet, I'd probably have to say Smash Bros - I was really surprised how much hype there is among the Nintendo fans. On the other hand, Mario Kart games are much more accessible and universal. Everybody and their wife loves a good, goofy racing game.

I'll say... Mario Kart. But it will be close.

^But think about this: Kart console series sales have gone DOWN, not UP like Smash series. Also, Brawl has much more hype surrounding it than Kart.

Wikipedia is pretty shitty for game sales info, due to their policies excluding most reliable sources. 

In any case, both Wiki and VG Chartz numbers have them so close to each to each other that it's impossible to say one definitely outsold the other.

I think Smash Bros. so far Smash Bros. is proving to bring alot more to the table, while Mario Kart seems to be pretty much the same experience. I know there will be online and more karts in races and such, but still Smash Bros. is on the rise and Mario Kart is on a slight decline(although this one should turn that around being on this hugely popular system.) Other reason is japan is big on the Wii along with America and they are the ones that bough more of Smash bros. over Kart last time, so I except with there huge popularity they'll do the same. Plus Smash Bros. is so versatile with controls, no one will complain about that. While I haven't personally been keen on driving titles with the Wii, in fact I probably will pass on MK.

Mario Kart by about 1 million, both will sell very well but overall I think Mario Kart will do better.

i think MK but I want Brawl

-Mario Kart 64 sold 9.9 millions on 33 millions Nintendo 64s.

-Double Dash sold 6.6 millions on 24 millions Gamecubes.

Now...

-Smash sold 5.5 millions and Melee 6.6 millions.

So, Smash series sales went UP despite the SMALLER install base. So my bet is still on Brawl.