Well, Overheat has 130 power, 90 accuracy, and only 5 PP. Flamethrower has 90 power, 100 accuracy, and 15 PP. Overheat lowers your Special Attack by 2 stages after attacking, whereas Flamethrower has a 10% chance of burning your opponent. These moves are both Fire-type and in the Special category. If you want more power, go with Overheat. But, if you want to burn the competition, I'd go with Flamethrower.