Dang! This here's a tough call? Two top contenders of fire going at it?
With Natsu Dragneel, he'll eat all types of fire. Not so-smart as he likes to fight & get 'fired up' in the heat of battle.
Then there's Portgas D. Ace, he's much wiser & can be careful to consider how to fight opponents when they pose a threat.
Another tricky thing, is that Ace can manipulate fire from his body & around outside sources, & Natsu can produce his own fire & eat fire to refuel (except his own?), so it's almost a basic stand-off? They can level a plain-field until they go toe-to-toe with their fist.
Man, it's just so crazy, one has to see how such a battle will go...