Well, when not thinking about your other moves, Throat Chop would be the better move because it gives STAB, has a higher base-power, and when looking at Incineroar's 4 weaknesses: Water, Fighting, Ground, and Rock, Throat Chop will be normal-effective against 3 out of 4, whereas Fire Fang is only normal-effective again 2 out the 4 (where each move-type is not effective to 0 out of the 4).
However, as you level up Incineroar, I recommend having both a stronger Dark-type and Fire-type move such as Darkest Lariat and Fire Punch. You should still have both a Fire and Dark type move in Incineroar's move-set in my opinion.
Your Choice! Hope I helped! :)