Given the set you mentioned in the comments to another members answer (reproduced below so that people do not have to scroll), Fire Punch is definitely the better choice. Against Aegislash# with 252 in both Defence and Special Defence Flamethrower will inflict 36.6 - 43.8% damage, whilst Fire Punch will cause between 43.1 - 51.1% damage. You would have to invest 140 EV's in SpA in order to achieve the same level of damage, which will obviously affect Electivire's Speed and Attack, so I wouldn't bother.
As an aside, have you considered using Thunder Punch instead of Discharge? I don't know the circumstances which you are using Electivire, but it is a more powerful choice (26.7 - 32% vs Discharge's 24 - 28.6% on the same Aegislash above.)
Electivire @ Expert Belt
Ability: Motor Drive
EVs: 252 Atk / 4 SpA / 252 Spe
- Cross Chop
- Fire Punch/flamethrower
# I know this is DPP but Aegislash has equal Def/SpDef and is weak to Fire so good test candidate.