I actually have this issue in GGXrd and UNIST. Deciding on which char to main is a bit hard, because almost both of them have the same principles. In my case, it's a bit hard to decide, because I'm trying to find a main that fits me well, according to what I have seen at these games.
In my case, in GGXrd, I'm trying to find out which should be my main between Faust, Jack-O and Chipp. Used to play Jam and Kum. And in UNIST, I'm trying to decide between Akatsuki, Enkidu and Hyde.
Usually, this problem isn't happening in some games, because most of us have already played these games' old installments in the past. Another way to decide the main is to find a character that you have higher success rate of landing even the complex combos.