XMPP - it's fully open, while (I think) Matrix is controlled by a single org

XMPP. Honestly I mostly use Delta Chat these days but XMPP is the fallback.

I don't like some stuff about matrix, mostly the Amdocs connections, the janky phone apps (incl element), the electron pc apps (incl element), and the Amdocs connections. Yeah I said it twice lol.

Which one would you pick?

Personally? I can use XMPP through emacs, so XMPP.

Go with XMPP. You already know the technical reasons—lighter, less metadata, older protocol with more time-tested decentralization. But heres the thing most people skip over: XMPP is philosophically simpler. Its designed to be federated from day one, like email. Matrix is building toward that, but theres still more of a "server as platform" assumption baked in.

For a friends-and-girlfriend group chat? They both work fine. But if youre already running your own infrastructure because you care about this stuff, XMPP is cleaner. The learning curve exists, but youre clearly technical enough to handle it.

One caveat: clients matter more with XMPP. Conversations, Gajim, Psi—pick one that actually gets updates. Matrix clients tend to be more uniformly polished.