Okay,lets compare them.
Lava Plume-80 Base power.
Effects:Lava Plume deals damage and has a 30% chance of burning the target.Hits all adjacent foes I double or triple battles.
Flamethrower:95 Base power
Effects:Flamethrower deals damage and has a 10% chance of burning the target.Hits a single foe in a double or triple battle.
Overall,i suggest you use lava plume only in single battles so that your own Pokemon doesent get hurt and get burned.
In double or triple battles,flamethrower is more reliable.
If you want use only one of the moves you said,flamethrower is my reliable.