Based on strength alone, I think Ace really is the better fighter, I'm not entirely sure though if Boa Marigold has no chance at all. One of the attacks she used against Luffy back then was the fire based Hebigami-Tsuki: Salamander after all. Whether she uses a match or Ace to light herself on fire shouldn't matter. It doesn't exactly help her against fire itself either though, so Ace likely wins - if he doesn't fall asleep and gets hit by the poison...