This is merely as a "differences" list, not a judgement on which one is "better" than the other.

FoV:
+ Inflicts Fire status on normal swings.
+ Hitboxes on melee swings more consistent(? Need data from Nitronicx for more deets about this)
+ Powerful Charge attack hits more times than calibur and deals more damage per hit, meaning it deals anywhere from 1k to 2k damage per charge, not counting Fire status.
+ Both charge attacks and normal attacks hit harder against Beasts and Gremlins (Charge attack dealing around ~700 damage to gremlins if hitting them from the charge at a 90 degree angle, and the added fire dealing higher DPS than base Combuster swings.)

? Easier to obtain... sorta (Combuster has to be crafted from 2* brandish, FoV is a boss reward from (arguably) the most ground boss)

- Self-Fire isn't a good thing
- Deals less damage per melee swing than Combuster, as well as having lower attack speed.
- Less "safe" than Combuster (which has higher attack speed, making it safer, as well as a more "controllable" charge attack)

Combuster:
+ Higher damage on melee against neutral and weak enemies.
+ Charge Attack is powerful against standard enemies (since they ride the whole wave of explosions)
+ Easier to use effectively

? Harder to obtain... sorta (Combuster has to be crafted from 2* brandish, FoV is a boss reward from (arguably) the most ground boss)
? Directly competes against the superior Acheron

- Much worse against turrets AND bosses.

Anything I missed?