The question of whether Duo or Trio is best in the long term is a very easy question to answer in my opinion, if you just look at fighting games historically.
What has been the consistent top tier characters "thing" across all fighting games? What have been the characters that are top tier early in a game's life cycle where there aren't a lot of truly skilled competitors?
The early top tier characters are usually ones that do a lot of easy damage, and are really easy to play. In unbalanced games this is generally also where one character just completely dominates other characters neutral to the point of character select invalidation, or have an extra bullshit option that other characters simply can't deal with. This, I don't think is the case in Skullgirls since this game is pretty damn well balanced. Most characters can deal with other characters fine, and those that can't can always pick an assist to make it better.
However, even in balanced games, better characters rise to the top. Generally, this is because the character does have -better- options than others while not being truly oppressive, is better at working through common situations the game can throw at them, and/or has slight advantages that top players get extreme mileage out of.
As neutral and setup efficiency climbs, the benefits of having, say, one neutral assist and also one setup assist will be obvious compared to JUST a setup assist or JUST a neutral assist. Playing a neutral-based team with two neutral assists that cover completely different areas will also be great for a more neutral-minded player or team.
As players get much better at conceptually using characters teams or strategies that require technical knowledge or technical precision to play, there will be much more growth with those characters/teams/strategies compared to much simpler characters that might hit their skill cap earlier.
@Dime_x mentioned something about the benefits of an extra assist being fleeting. This is true. This is also true of spacing and situational advantage. Good players will jockey for good and advantageous positioning and on read good positioning can be both claimed and taken away with fast and accurate movement. Being able to not only win when you have easily reproducible large situations of advantage, but really small windows of advantage becomes much more important as skill levels rise.
Therefore, even if having an extra assist might be a fleeting advantage, it is a fleeting advantage that when properly used will get you the hit or better situation and win you games. Objectively more opportunities and methods to create a win than a Duo team.
Picking two easy/straightforward characters on a duo with decent assists will make a good player great in the short term since they will just destroy everyone else that's under them with no problem, even in the case of going up against superior teams. This will not last, unless the players are Japan-tier character and game loyalists.
tl;dr skill exists pls discover
