Which is better and why? Or do fire druids actively use both?

They use both. Armageddon runs for a long time, so you actiave it and it keep raining firey bs from the sky. They then use a combination of Volcano/molten boulder and fissure. where fissure gets not used on fire immunes.

Volcano let's you place it further away, Armageddon seems like for closer encounters as a defensive tool until you're more prepared for aggro.

I feel like Druids just use their 3-4 skills in rotation to maximize damage and AoE.