I prefer Supers over Competitions for fighters, but to be honest, I'm not a big fighter guy. That is why I would go with concave buttons if I were you. You'd really want to get the opinions of hardcore fighter fans though. I know a lot of people who play fighters like Sanwa joysticks and convex buttons, so that might be the "thing to have" for fighters.
For me, a casual fighter fan, I find Supers to be fine.