I never used it (Jellyfish) either. I find Shadowbat the best for guerilla tactics and for the tactical options it provides.
Jellyfish is also the slowest of the Gray codex ships, although there are uses as a meat-shield due to the shields.
Then again, Snake is faster and has shields.
I use Fortuna when Shadowbat is not an option (space missions, house raids, sea missions etc.), although tactically it's terrible leaving your units exposed from the get-go (no doors). At least it has a good amount of crew space (12) with 2 aux slots.