Now this is an interesting match. Both of them have basically the same AP, so it comes down to a battle of skill and powers. Both fighters are very experienced combatants and have fought several powerful opponents throughout the years. They both possess Instinctive Reaction and an ability to analyze the fighting styles of opponents after observing their movements, and both can increase their power in a fit of rage. Both of them also possess a mastery over their respective powers and have a variety of named moves which utilizes their abilities in different ways.
Natsu's flames could certainly be a problem, and his AOE would also make it difficult for Luffy to dodge, not to mention he can also do pinpoint attacks from a long range distance with basically perfect accuracy. Natsu can also increase the temperature of his flames as he experiences a surge of emotion, which could be a big problem for Luffy to the point he'd eventually be overwhelmed and potentially disintegrated by his flames if he's not careful.
Then there's Luffy. His rubber powers could let him play the long-range game as well. A few big advantages that Luffy has is that he has become very experienced with Haki. Conqueror's Haki isn't really useful here, since Natsu would undoubtedly be able to resist its effects due to his own strong will. However, Kenbunshoku Haki would allow him to predict Natsu's attacks with greater accuracy, and Busoshoku Haki would allow him to deal more damage with his attacks. Something else that's probably been forgotten is that Luffy has 7-A durability, which would allow him to endure more of Natsu's attacks than Natsu could against Luffy's attacks, though since the AP gap between City and Mountain level is minimal, Natsu could still harm Luffy with his attacks, it would just take longer. Luffy also has Resistance to Heat. I'm not sure the extent of the temperature he can endure, but it would at least be enough for him to not be bothered by Natsu's regular flames. The Reactive Power Level that allows Luffy to evolve as the battle progresses would be a big advantage for Luffy.
Honestly, I'm not quite sure who would win. I'd like to hear other people's opinions.
