Who should I rank up for the x-men sos objective?

Apparently my x-men roster is very limited, so I'm either going to need to use my 6* r3 gambit (who I've tried and he's terrible because you have to do a lot of blocking to build charges) or I need to rank up someone. My options are a 5* r3 sig 40 storm px, 6* r1 unduped professor x, or 6* r1 unduped white magneto. I leveled up professor x a while ago, but I don't like him, which is why he's still at rank 1. I would be taking storm px to rank 5 and most likely the 6*s to rank 2 because I'd really only be using whoever I rank up for this fight. Comment if I should rank them up further.