The next guy's job is to tell you XMPP is lighter, gen-er-ally viewed as simpler, with a wide array of clients and servers, optional encryption, and with a longer history (with that being viewed as rhyming with reliable).
My "job" in this holy-war thread is to tell you Matrix has become lighter over time, the "default" server Synapse has less, but IMO more up-to-date documentation with a real corporation behind keeping it up-to-date and useful, has a blossoming ecosystem of clients, servers, and bridges (allowing you to use it for other chat systems like Whatsapp and Telegram), has encryption being an enforced default for one-to-one mesasges (instead of XMPP's bolted-on after-the-fact extension), and a paid team to make Synapse more robust, reliable, lighter, faster, and more secure.
Take both arguments with a grain of salt, as I am biased as hell (to the point of donating a small amount monthly to Matrix, and starting flame wars like this one).
Thank you xethos, this has been exactly the kind of opinion and experience report that I've been looking for.
the one thing I really want beyond persistent over whatsapp is threads.
I hope matrix has a similarly trivial app/web/pc story as mattermost has, because the other users are not necessarily able to handle anything more complex than "download an app and sign in".
