In terms of strength and speed Luffy tops Natsu. But in terms of destructive ability Natsu is obviously over him. THe question is whether Natsu can survive a serious assault from Luffy. If he can then there's a good chance that Natsu fries Luffy. If not then it's Luffy's win.
On the other hand G2/3 Luffy vs DF Natsu is different. Natsu finally have some speed(still not enough to properly catch up) and Luffy isn't fireproof.
@battleheiz: He can. Natsu just needs to consume a flame with enough magic power. See the battle with Zero for example. He needed Dragonforce and Gerard made Natsu eat the Flame of Retribution to induce DF mode. On the other hand it's heavily dubious that Gerard can move as fast as a real meteor. Fast, but not that fast. At least from showings. Same way I don't believe Luffy's crazy Gear 2 calcs that OBD spams. They take everything as absolute and believe that calcs are telling the truth, even if there are contradicting situations.
I love luffy guys [ no homo] but still i give this to natsu he is weakened by fire due to him being made of rubber I know luffy would get a few hits in using sure will but Natsu is strong enough to take each hit