By the way, I forgot to mention, unless I have Heatran out (or you're anticipating a Heatran switch in) Ninetales should be using Heat wave on Hitmontop.
Extrasensory's Power vs Hitmontop = 80 x 2 = 160.
(Sunny) STAB Heat wave's Power vs Hitmontop = 1.5 x 1.5 x 0.75 x 100 = 168, and it hits both targets.
So Heatwave is better, unless you're hoping for the flinch chance too, or unless you have an expert belt (but... I'm not sure you don't, because fire + grass + psychic isn't exactly amazing for super effective coverage).