I was thinking about both, and to be frank, she can't control her quirk like Endeavor could, which was the only reason he wore a costume like that, because he could control where the fire would come out of, and something fire 'resistant' wouldn't hold up to exposed flames for a long period of time, so I guess going costume-less would probably better suit her quirk. 

Also partly that reason seeing as though I can't really picture a costume befitting someone like the Human Torch.