Like many have already stated, it's all about the bundles. If Ninty offers a MK8 bundle (still praying for a price cut with it) and actually makes a damn effort to properly advertise the game, MK8 will outsell NSMBU.

Probably Mario Kart 8 if they bundle it for long enough.

If Mario Kart 8 gets a bundle then it will probably outsell NSMBU.

Whichever game is bundled more xD

The thing is I think Mk8 will only enhance the sales of NSMBU, especially for a while so if MK8 does sell more than NSMBU I think it will have to sell at least 5-6million in order to tie it let alone beat it.

NSMBU if you include luigi.

Kart will probably sell great but if it doesn't give a big boost to hardware sales, I think it just won't be popular long enough to catch up to 2D mario.

Of course Mario Kart.

NSMBU's attach rate, on it's own, before the bundle that came almost a year later, was fairly impressive, over 50%.

But in this case I don't think it's much of a contest. U was a great game, but people were kind of burnt out, thanks mostly to NSMB2 launching on 3DS WAY to close to Wii U's launch, on the 2D Mario thing. Whereas people are genuinely excited about Mario Kart, and only one MK seems to ever launch (which is good) per system. There will inevitibly be a MK8 bundle, but even if there wasn't, MK8 would still soar in sales regardless.

MK8 will blow all previous Wii U games to date out of the water, for a plethora of reasons.

MK8

MK8. Both series have mass market appeal, but the difference this time is that MK8 is everything that fans want from a modern Nintendo title, while NSMBU (like 3DW) was viewed as a bit of an uninspired effort. The hype is far greater for Kart, to go along with the large fan base.