There won't be any huge divide between 3DS/Wii U players, especially since a lot of people want both. 3DS/Wii U will have the exact same meta game and balancing in place, so there will not be any divide. The thing about 3DS/Wii U is that they are different games in their own right, but having both will mean the best of both worlds. In terms of tounaments, the Wii U version will be dominant, but I don't think this will bother 3DS players. They get to use the same characters and balancing, but on a HD screen and using a better controller. Any training done in the 3DS version will carry over to the Wii U, so people can still perform well in Wii U tournaments even if they only own the 3DS version. The reason why Melee and Brawl had a divide was because Melee players didn't like the fact that Brawl was dumbed down as a competitive game. And then Brawl players didn't like it when Melee players were calling the ENTIRETY of Brawl a crappy game. The two games played differently, but 3DS/Wii U will play the same.
Really, it depends on the individual. I consider myself a "Smash Bros fan". That means that I will get every game and probably like each one. Overall, I prefer Brawl to Melee, as it had so much more content, from the new characters and stages to the Subspace Emissary. BUT, I do feel Melee is a better competitive game and still play it for that (even with P:M's existence, I still come back to Melee for that Falco).
If Smash Wii U/3DS manages to top Brawl for overall fun factor and longevity, they will be my new favourites. I think Smash Wii U/3DS already looks better than Brawl in terms of overall mechanics (hitstun appears to be back, game has a faster pace, custom movesets), so there's that. Online also looks excellent. Now I just need to see if it will have any huge new features. 3DS has Smash Run so far, so it's looking good.
However, some people are just "Melee fans". They are at a point where they don't play the game because of the Nintendo fan service, they play it for the mechanics and nothing else and because of the competitive scene that has developed. That's completely fine. Heck, I'm sure there are more than a few Melee fans who aren't really Nintendo fans, and would likely play the game without the Nintendo atmosphere. Personally, the fan service and atmosphere is part of what makes Smash Bros great to me. It's definitely what drew me in. I can't imagine it would be the same if it ended up with the generic martial artists it was originally going to have.