@Nibbler3100
For now, we're using their current stats, both of them being 7-A. We're also assuming speed is equalized anyways. Until the upgrade happens, that would be the case.
@GoldTempest
Sanji's only shown that ability against two people so far, the chief on the train to Enies Lobby and Duval. Both of whom were weaker than Sanji. He's never shown that he could repeat that stunt on a character with equal durability to himself, like Natsu in this case.
@Delta3000
I would argue against Franmalth's weakness being "obvious". There are also other circumstances such as the fight against Yuka, Natsu being able to read both the fighting styles of Sting and Rogue down to their very footwork and completely dominate the two, being able to dodge Mard Geer's thorns even though he had never seen the curse before. My main point being that if Natsu did lose this fight, it wouldn't be because Sanji was more skilled/more adaptable, because Natsu has shown in many cases he can adjust to defeat a variety of different opponents. This would make sense, given that Natsu lives in a world filled with mages that all have different offensive magic and hax abilities-to survive as a guild wizard, he would have to be adaptable. Sanji could possibly be more skilled at hand to hand (a bit of a misnomer in Sanji's case lol, more like foot to foot) because that is his entire fighting style, but since Natsu has access to magic as well as hand to hand that would more than even things out on that ground alone.
The way I'm leaning towards right now is Sanji taking this fight at high-difficulty. Both fighters are adaptable and skilled in close quarters, but since Sanij has precog in the form of Haki, he has a slight edge. What would make things difficult for him is the nature of Natsu's magic. Sanji has to get in close to damage Natsu, while Natsu has ranged attacks as well as melee attacks. The heat which current Natsu emitts even passively when he powers up is well above temperatures emitted by combatants in the One Piece verse, except possibily Akainu. Sanji has shown some resistance to high temperatures, but not to anything on the level of Natsu's fire. So to attack Natsu, Sanji has to enter a range of temperature which would practically roast him alive. The only reason I think Sanji can still win this is due to his Armament Haki, which could protect him to some degree, at least while he's attacking. Natsu is stubborn and extremely hard to take down, so even with precog Sanji is likely to take some hits, especially if he's being worn down by the heat. It would still be incredibly difficult, but Sanji's Haki is what makes me think he could win after a tough fight.