I think the really big picture is that Pleroma can actually serve as a competent backend for *anything* you want to build on ActivityPub, while Mastodon is pretty much stuck in the microblogging space.
All you need to bring is the frontend of your choice really.
@lain Maybe you should also mention that #Pleroma development is done on own #GitLab instance, and not #GitHub. — That's an advantage, IMO. So I'm intrigued.