Normally I'd just get Smash Bros for the Wii U but I don't have one. I'm also broke and don't plan on buying one. I will buy one of these, just can't decide.

USFIV:

Pros:
-New Characters
-More stages
-Omega Mode
-All Costumes

Cons:
-All new modes are online only
-It's the same game as before (I have Super Street Fight IV)
-No real content if you aren't planing on going online.

SSB3DS

Pros:
-It's a classic time killer
-Online is free
-It's Smash Bros.

Cons:
-Performs better on a New 3DS which I don't have.
-Control issues (from uncomfortable to c stick breakage)
-Shallow content.

Well yeah, I do have SF, but this one has a lot of extra characters. Typical Capcom that they sell you a full game 6 years after release.