IMO Serissu's ability waaay better then Howlrunner's because green dice are much less consistent, and you'll need that re-roll more often.
Serrisu's ability doesn't scale as well. Less ships can take advantage of Serissu's ability every round than they can Howl's and you have no control over whether you get to use her ability in a game, your opponent makes that decision. Serrisu's ability is nice when you've got one or two ships that are more expensive than she is. It's not so hot in a swarm of cheap ships. She's a soft Biggs.