RPGMemes

I think the difference really comes if fireball would reliable actually kill your opponents and if that's your objective.

A phalanx of hobgoblins have 11 hitpoints each, so their half damage on a success is still likely to mean they go from full to dead in a fireball. Hypnotic pattern would probably still take out 3/4+ of them, but now you and mop up the incapacitated creatures.

Hypnotic pattern is more versatile as it's humane and can be used to achieve multiple objectives, but when it's life or death, nothing kills like fireball.