Ok, So, I'm assuming you have 31 IVs in both stats. I'm going to do the calculations on a boufallant, because it has equal defense and special defense.
For the first calculation, I'm going to use 252 in HP and 252 Defense + a defense increasing nature, and I will be using 252 atk EVs and an adamant nature on flareon, while using fire fang.
252+ Atk Flareon Fire Fang vs. 252 HP / 252+ Def Bouffalant: 87-103 (22 - 26.1%) -- 7.5% chance to 4HKO
Now, here are the same calcs, however, flareon is running 252 Special attack EVs and a modest nature, and Boufallant is running a Special Defense increasing nature and 252 Special Defense EVs instead of 252 Defense EVs.
252+ SpA Flareon Fire Blast vs. 252 HP / 252+ SpD Bouffalant: 118-141 (29.9 - 35.7%) -- 35.1% chance to 3HKO
So, I actually got that fire blast is more powerful on flareon than fire fang, which is interesting, but not surprising when you look at the difference between their base powers.
Hope I helped :)
EDIT: I thought about AUG's comment. If accuracy is a problem you find yourself in, go with fire power, but based off of my calculations, fire blast does more because it's just a nuke.