Although it is contradictory to what logic would dictate, Fire Blast is the superior option for surpassing the physically inclined steels that stand in your way
Here are some calcs of fire fang vs. fire blast's power against the Pokemon you would use them against
- 0- SpA Garchomp Fire Blast vs. 224 HP / 32 SpD Skarmory: 166-196 (50.76 - 59.93%) -- 85.94% chance to 2HKO
- 252 Atk Choice Band Garchomp Fire Fang vs. 224 HP / 252+ Def Skarmory: 122-144 (37.3 - 44.03%) -- 99.95% chance to 3HKO
- 0- SpA Garchomp Fire Blast vs. 252 HP / 4 SpD Forretress: 384-456 (108.47 - 128.81%) -- guaranteed OHKO
- 252 Atk Choice Band Garchomp Fire Fang vs. 252 HP / 252+ Def Forretress: 244-288 (68.92 - 81.35%) -- guaranteed 2HKO
- 0- SpA Garchomp Fire Blast vs. 252 HP / 208 SpD Ferrothorn: 192-228 (54.54 - 64.77%) -- guaranteed 2HKO
- 252 Atk Choice Band Garchomp Fire Fang vs. 252 HP / 48+ Def Ferrothorn: 296-352 (84.09 - 100%) -- 6.25% chance to OHKO
Those are three of the most commonly used walls that resist dragon that are not weak to ground. These Pokemon, that are resistant to both dragon and ground, are really the only Pokemon you should use this fire coverage move against. For Grass types, and bug types, a STAB outrage will have 180 base power, as opposed to 130 from fire fang and a 240 from fire blast coming of a weakened special attack stat. Even dragon claw does a strong, reliable 120 damage. As such, Fire Blast, a move who does what the fire coverage should do better than fire fang, should be considered a superior option.
However, if you find the fire blast is not satisfactory, do not feel the need to avoid using fire fang, as it is still a legitimate option.
Hope this helps