There's almost no difference between both moves, they have similar damage outputs in their normal targets.I started using a Fire Blast Slowbro and wouldn't you know it, I missed the first FB I threw out! Is FB needed for something or will Flamethrower work? I know that both 2HKO Ferrothorn but is there something I'm not thinking of?
0 SpA Slowbro Fire Blast vs. 252 HP / 252+ SpD Skarmory: 144-170 (43.1 - 50.8%) -- 3.9% chance to 2HKO
^ Specially defensive
0 SpA Slowbro Fire Blast vs. 252 HP / 4 SpD Skarmory: 212-250 (63.4 - 74.8%) -- guaranteed 2HKO after Leftovers recovery
^Physically defensive
0 SpA Slowbro Flamethrower vs. 252 HP / 252+ SpD Skarmory: 118-140 (35.3 - 41.9%) -- 86.5% chance to 3HKO after Leftovers recovery
0 SpA Slowbro Flamethrower vs. 252 HP / 4 SpD Skarmory: 172-204 (51.4 - 61%) -- 94.1% chance to 2HKO after Leftovers recovery
0 SpA Slowbro Flamethrower vs. 252 HP / 168 SpD Ferrothorn: 200-236 (56.8 - 67%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Slowbro Fire Blast vs. 252 HP / 168 SpD Ferrothorn: 244-288 (69.3 - 81.8%) -- guaranteed 2HKO after Leftovers recovery
^ Specially defensive
0 SpA Slowbro Fire Blast vs. 252 HP / 4 SpD Skarmory: 212-250 (63.4 - 74.8%) -- guaranteed 2HKO after Leftovers recovery
^Physically defensive
0 SpA Slowbro Flamethrower vs. 252 HP / 252+ SpD Skarmory: 118-140 (35.3 - 41.9%) -- 86.5% chance to 3HKO after Leftovers recovery
0 SpA Slowbro Flamethrower vs. 252 HP / 4 SpD Skarmory: 172-204 (51.4 - 61%) -- 94.1% chance to 2HKO after Leftovers recovery
0 SpA Slowbro Flamethrower vs. 252 HP / 168 SpD Ferrothorn: 200-236 (56.8 - 67%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Slowbro Fire Blast vs. 252 HP / 168 SpD Ferrothorn: 244-288 (69.3 - 81.8%) -- guaranteed 2HKO after Leftovers recovery
There are more targets, but i think you know wich are those.