For this I would have to say Luffy. The only one of Luffys attacks that wouldn't work on Natsu would probably be Red Hawk, which involves fire. Luffy is much faster and more durable, I'm not saying that Natsu can't take a hit, but Luffy is rubber, it wouldn't help him with the fire aspect, but it would help him with the physical hits. Plus, with Luffys Gear 4 "Bounce Man" is super op. He's kicking the air so fast hes actually flying, which gives him more mobility than Natsu, and his haki is stronger that Natsus dragon scales. All in all, I think Luffy would win.