Like Matrix the clients aren’t all equivalent without feature parity (& no concept of the flagship or implementation client). For desktop, Gajim has the most power user features but issues rendering in smaller windows like a tiling split (& being written in Python has other issues). Dino is feature-complete & calls tend to always work—great if not connected to tons of chats. Profanity is the best TUI which is very fast but usability is really good for some things & really bad for others (like accepting no OMEMO keys). I use all three depending on the environment & task. Android it is a lot clearer where Cheogram takes the cake for me being a Conversations fork but with OLED black support as well as webxdc. For the web, Movim has the best UX/feature set & can be used anywhere a browser can with PWA support. You can also just check to see what provide OMEMO: https://omemo.top/.

ActivityPub is a JSON-based protocol for seems primarily built for social networks, with the DMing experience normally not being secure or particular fast. XMPP is largely for building networks for passing messages & client presence—which can be extended to support PubSub like MQTT. It isn’t normally built for social networks but Movim & Libervia have extended XMPP to be a social network.