For users point of view this spreadsheet comparing pleroma and mastodon has been done in July 2019 (mostly in french) there: https://lite.framacalc.org/masto-plero, with a linked blog post (french) explaining it a bit there: https://lord.re/posts/185-tableau-comparatif-mastodon-pleroma/

Pleroma is community-run, meaning that a decision isn’t taken by one person

Pleroma is very flexible, a bit too much sometimes from a developer/maintainer point of view but it’s really nice to have from an admin point of view

Pleroma is actually using ActivityStreams in it’s database so it is future-proof and can evolve to non-microblogging (I actually quite want to replace peertube’s backend with pleroma after having tried to host a peertube instance), Mastodon is still storing it in a typical GnuSocial/OStatus way.

Mastodon has a very poor documentation, most of the API endpoints are lightly described in their Pull Requests, and for reimplementing the API in pleroma I basically always reversed it and then looked at the bits of documentation (when it was present…) to confirm my ideas

Pleroma is/was done mostly from fediverse users that came years before Mastodon, so it isn’t a mastodon clone but is quite related (a lot is taken from classic Twitter, Mastodon is from modern twitter).

Also this is quite offtopic but I think there must be more experimental implementations in the fediverse, I think pleroma has quite done it’s experimentations, stuff added will be explored but I feel like there is a lot of things we can’t do and sadly a lot of new implementations are clones.