For those saying the 3DS will be the same as the Wii U for practicing advanced techniques, already in the demo this has been proven false. There is a way to pivot smash that isn't possible in its entirety on the 3DS version due to the controls. There is actually the possibility that the 3DS AND the Wii U could have console exclusive ATs.
I don't really see a reason for it to take off, once the Wii U version is out there won't be any reason to bring forth the 3DS version at tournaments outside of easier friendlies, there's just no benefit hosting tourneys for the 3DS version over the Wii U version. It's easier to set-up just a few Wii Us, it's easier to stream from a home console, it has Gamecube Controller support, it has the best stages for competitive play thus far, not to mention it also has the better graphics which is nice for the stream. Because of this I think the 3DS version is bound to just be the casual experience or for training yourself when you don't have access to your Wii U, it's simply not going to be a tournament hit.
Due to logistics it may not be possible to allow customization within reason on the Wii U version. The 3DS allowing those to bring their own consoles with their own preset data not needing to be transferred from console to console may make customized tournaments the standard on the 3DS while the Wii U uses standard moves and the like. As a LARGE portion of the scene seems to just wanna ban Miis and only use standard movesets, this could make a compromise so we could have a scene for both. The fact that the two games could have a separate meta game is enough for them both to have tournaments.
How's about how much easier, faster, and better tournaments will be able to be on the 3DS? With Swiss brackets possible, not only do new players (and the pot fillers) get more games and better games to their skill level so they can have close matches then pools allows, but a Swiss Tournament could manage large events WAY faster, even with a cut to a top 8 or so for double elimination. You could host some of our larger tournaments that take an entire day in a matter of hours. Events that go way past when they are supposed to end wont happen. You could host a 50 man event on a weekday night and still be able to get everyone home in time to get sleep for work or school in the morning! Tournaments not limited to weekends is AMAZING. If Magic the Gathering can host events at that size with a game that generally take quite a bit loner then most of our sets on weekdays and have people show, we can do it with Smash 3DS. You can't even get CLOSE to managing that with the Wii U where even a 50 person event can take up a huge portion of the day, players get to play less matches, AND friendlies are more difficult to play.
By the way in terms of streaming, if the Wifi is good enough, someone living thousands of miles away could stream matches for events they could never even drive to especially considering some events with even local connections may have tiny bits of interference to the point where Wifi play may be 100% lag wise for tournament practice and use. . We did things like this with PSASBR, the game with netcode worse then Brawl. A few dedicated streamers could stream tons of 3DS events and cover way more then anyone with console streaming equipment could.
Plus with the WAY larger install base for the 3DS it's not going to be hard to find someone to play smash with. There will be enough folks ESPECIALLY if we reach out to newcomers and bring new competitors into the scene to easily support smash 3DS.
All it takes are people to support it. Brawl isn't dieing because of game mechanics or boredom with the game, it's dieing because there is no one dedicated to hosting it, almost no one ever streaming it, and no one willing to work for it. Even Smash 64 has more streams and dedicated folks then Brawl! Melee is exploding because of the huge dedicated fan base that's constantly driving it forward. If there are those willing to stand behind it and work to make it happen the 3DS WILL have a scene to rival and possibly even eclipse the Wii U.
I know I'm one of those folks. I put the money in to be able to stream the game, I'm making a website that will have tools to help those hosting events online. I'm working with a group to create DataKae, a data collection system for smash events so our rulesets and knowledge of the game will be better then ever before. People on here have asked for a way to have clans, I'm willing to work to make a great system for it and provide it. I'll be hosting and streaming online events every week so people have a place to play. I'm just ONE PERSON. Imagine if even a few more people jumped in and helped!
The only thing stopping the 3DS from having a competitive scene are the people who love smash who want to say it doesn't have a chance before it even gets rolling. Every time someone bashes the 3DS and says it's not worth even trying for you are just driving away potential people from the smash scene. Stop it. You'll do to the 3DS what Melee players did to Brawl. It's already happening, so why not nip it in the bud and accept that both version deserve space and can have phenomenal scenes?
Wall of text, but worth reading for sure.
Anyways in terms of stages there is a legality speculation thread. I'll bump it up and tag some of you folks in it so we can talk in the right place. Or you can always come over to the 3DSA, we'll be glad to discuss it!
) and uncomfortable controls will turn people off to playing the game super seriously. It'll be a thing for the few months it has before the Wii U release, however, once the Wii U version drops, the 3DS will just be for fun in my opinion. It would be a lot to have Tournament Organizers squeeze in the 3DS version while including Smash Wii U, Project M, and Melee. Although all three of those titles won't be played at every single tournament, I feel as though the scene for those games will still be strong at the same time. 
