Fire and Corrosive are both good for their respective enemy health, what you really want is for them to roll with increased projectile speed and faster charge time, given how the projectiles spread and move.
I feel like it depends on which character. I go for an elemental Amara build: shock cutsman, Otto Idol and transformer. That way its superior than fire. Havent gotten my hands on a corrosive one but i guess for my build shock is the best Option.
i usually use a shock dictator for shields, or shock cutsman, but i like my fire cutsman the most, heck i like it for every type of health bar cus it still melts shields.