only time I've seen this event. guess which option I choose.

Upvote for gnome

Well, if you choose to save the Slugs, when you destroy the Mantis ship, the Slugs will try to back off the deal... and you can choose to destroy them - no battle. Either the Slugs didn't really have any rewards to spare, and you take the scrap from their scrapped ship, or they relent and give you a free item.

I never pass up the chance to get a Mantis on my ship. I also tend to make bad choices in this game. Coincidence?

I think I always chose attacking mantis, because their ships usually don't have threatening weapons (early on) and are easy to deal with if you can negate or deal with teleporting. On other hand, slugs tend to have more shields (up to 5) and more weapons, and are quite a pain to fight.

Same goes with Zoltan elder that asks you to choose your undoing, I always go for Mantis over Rock and Slug.