SSB 3DS & Wii U are not really that different. The major difference is the fact one brings back a bunch of Brawl modes with a crappy "party" mode and the other lacks tons of features and has a poor man's version of the temple labyrinth from Melee's Adventure Mode. The stages are different, but this game is adding new stages as well.
For what it's worth, we already have confirmation of 3 new characters, at least 1 new stage (likely 2 for TWW), 1 new boss, and a new story mode section. It also has implications of new weapons, costumes & Adventure Maps to compensate for the new characters and potentially even the DLC characters that were shafted in the Wii U version.
Not to mention the fact the game now includes the ability to switch between multiple characters in-battle instead of just playing one. Who knows what else has been changed?
Depending on how much stuff it actually adds, hell, maybe it will end up worth it for the new content alone if it'd cost just as much to add it to Wii U without it.
I'd love to be able to have the characters, but the biggest problem with the DLC characters in the Wii U game is the fact that you can't really use them. They don't unlock anything in Adventure Mode, so you have to play all the base characters anyway. The 3DS version could finally give them stages where they are required to play and will get some light. Not to mention, I'd want the new stages & the switch ability too.