I've seen a lot of hype and I've might even seen some people claiming that. But are there really people who honestly believe this? And if so, why?

Looking at the sales data of both series it seems absolutely impossible.

I'm talking of course about the Wii U version.

Yes, I believe that Smash will outsell Mario Kart 8. Reason for that is because Super Smash Bros. even for both 3DS and Wii U has been out there very quite well. There has been competitions already and it hasnt even fully came out yet. The game has been playable in many booths like at E3, Best Buy, SCCD(sp?) and so much more. And plus thats not the full version that they are playing. Imagine playing it when it's launched, more feature will be available to play comparing to those demos at the booth/competition.
This will have a defiantly nice boost for the Wii U. Im guessing it will sell 1million+ units within the first week of launch same as MK8 did.

It might. SSBM outsold MKDD on the GC, so its possible.

I think MK8 will sell better overall though

Smash will do well like it always does but It wont sell more than Mario Kart. Mario Kart is a much bigger franchise than smash and will probably be the best selling game on the Wii U in its lifetime.

However I think Smash will push a similar amount of Wii U's as Mario Kart did and it has the benefit of selling this holiday so it will have big launch sales. I don't really here people say that it will sell better than Mario Kart but I here more that it will sell similarly to it.

Well, Melee did sell a little more than Double Dash on Gamecube. Melee was on the market longer but Double Dash was bundled at one point.

I think MK8 will sell more than Smash U but it'll be much, much closer than it was on Wii. Plus, Amiibos could prove to be a wild card in favor of Smash.

I don't think it will. Mario Kart 8 sold just under 3 million...in one freakin' month...in the middle of summer. That's impressive. I think both will sell well though.

It won't. The only Smash to outsell a MK was Melee vs. Double Dash, and that was by barely 200k. Mario Kart has always been a more popular series, and will have better legs than Smash will.

Lifetime? No. 1st month sales? Quite possible.

No wai!! I think Mk8 appeals to more people than Smash Does... For better or for worse

SSB will push 5x more wiiu's then MK8, will it outsell it lifetime? No. I think SSB will sell 5mil-6mil and mk8 will do 7mil-8mil (all depends on how much wiiu installbase will be