Aside from "XMPP is old" I don't see any reason to favour Matrix. I also don't see XMPP being around long enough to have a strong ecosystem that's constantly built on to be a bad thing.
From a #selfhosting perspective XMPP is far easier to run as well.
For E2EE they both use OMEMO.
And ofc it's in the name, XMPP is extensible.
Technically speaking I should say XMPP can and often does use OMEMO but can be used to implement literally any type of E2EE you want. You can outright port the full Signal Protocol to it and encrypt metadata if you modify the code a bit. You can even use PGP (although I view this as a legacy feature when OMEMO is more features, uses only ECC, and provides forward security).
@letterus if as you yourself just said Matrix is one company making everything, wouldn't that be a point against Matrix being some kind of community lead ecosystem? Furthermore your statement about XMPP make it seem like it has much more claim to being an open ecosystem as opposed to one company just making everything.
A federated social network for aspies and other NDs to be themselves. Chat to others with ASD, ADHD, etc. Connect to the fediverse to bring in content from a whole network of niche sites and follow people who share your interests. Strict yet flexible privacy controls for your content with no ads or tracking.