Nintendo's latest financial results are in and they contain the total shipment data of MK8 & Smash Wii u as of the 3rd of January 2015. Heres how they compare to vgchartz data as of the 3rd of january 2015 as well. Shipment data also includes digital.

Mario kart 8 units shipped 4.77m , vgchartz has it at 4.12m sold.

Super Smash bro's wii u units shipped 3.39m , vghartz has it at 2.91m sold.

The Wii u is also comfirmed to be at 9.2m units shipped, that puts MK8's shipped attache rate at 52% and Smash Wii u's at 37%. MK 8 was released on the 31st of may 2014 and Smash Wii u on the 21st of november 2014. Mk8 has been on the market for 31 weeks and Smash Wii u has been on the market for 6 weeks. I think both games are selling amazingly considerlng they are limited to  a relatively small install base. I'm pretty sure that these will be the top two biggest sellers on the Wii u durings its lifetime. Which game do you think will come out on top?

MK8... Mario kart is bigger than smash

MK8 - by a good amount

Likely MK8, but hey, Smash ain't doing bad either, and both are excellent games.

Mario Kart 8

Mario Kart. Smash may have a bigger first year, but Mario Kart's legs are infinite.

I'm sure It will be alot closer than you think.

difficult to decide, i have played both and cant say one is better than the other in gameplay excperience, both are enjoyable and addictive

You're comparing an apple to a slightly bigger apple.

There was never a chance for SSB.