What I like about the 3DS version, and also what makes me prefer it over the Wii U version, is the convenience. To play the Wii U version I have to go to the living room, turn on the television, turn on the Wii U, pick up the Gamepad, boot the game (for some reason, it takes longer than it should to boot) and then play.
To play Smash 3DS, all I need to do is open the 3DS and play, because thanks to sleep mode, I don't need to close the game/turn off the system. I can't even remember the last time I turned off my 3DS. Plus, since the 3DS is really small, I carry it with me to a lot of places, so I can usually play one or two online matches when I am bored, then close the system, then play it again in a few hours and so it goes on.
So I love the fact Smash 3DS exists, so much that people saying it shouldn't have been developed/it is/was a waste of resources used to truly annoy me. The 3DS version might not be useful for le competitive tournament players, but for a lot of others it's really great (otherwise, the online would be dead by now, but it isn't). Personally, I have 3x more time clocked on said version if compared to Smash Wii U and I doubt I would have played Smash 4 as much I have so far if the 3DS version wasn't a thing.