Pros:
- Option to play SSB earlier than expected, on 3DS.
- 3DS version saves money for 3DS owners that don't want a Wii U.

Cons:
- You need a 3DS if you wanna play SSB early.
- 3DS version is 1 player per handheld

People like to forget the main draw for a console Smash (or Mario Kart). Nobody cares about the resolution upgrade. It's the fact the in order to have a satisfying local co-op with the 3DS version, you need 4 3DS's and 4 copies of Smash 3DS amongst 4 friends. With the Wii U version, you simply need one game, one console, and for controllers, which most people invest in anyway. It's a far more convenient way to play Smash or Mario Kart.

If you have friends, that is.

 Smash Bros 4 releasing first on the 3DS will most likely affect the Wii U version's sales, which is a shame, because the Wii U version will have the better stage selection ( in my opinion at least) and a better online component!

I completely forgot to consider the factor of price.  The 3DS version will be cheaper as well.  While the Wii U will have unique features, one has to also realize the 3DS version also has unique features.  When you consider that the 3DS version is most likely $10 cheaper (and the system is cheaper, too!), there's a good chance that the Wii U version will look "expensive" in comparison.

I'm worried people who aren't already fans who want to double dip will look at the Wii U version, even if they own a Wii U, and think "will I pay $10 more for [X] mode and HD graphics?".

Oh well, like I said, we'll know the reality of the situation almost a year from now, no point in really saying much more on the matter!