XMPP is a simpler system that works on a "need to know" basis, meaning that servers and clients more or less only get the (meta) data that they need to function but not more. A group-chat for example exists only on one server and chat-history is only shared on request and usually is deleted quite quickly (that depends on the server settings though).

Matrix on the other hand works on a "replicate everything" basis (to be more resilient to censorship and other network disruptions). For that it runs a highly complicated distributed database where all (meta) data is automatically shared with all participating servers and is stored indefinitely. This has its upsides and downsides, but from a privacy/security perspective this is a strict negative.

I don't think it's better, but it's certainly different. For example, the XMPP ecosystem is mostly not based on web technologies, which makes incentives to build lighter clients: they have less attack surface, and arguably less unexpected fingerprint/leaks than a modern web browser engine.

Still, there's plenty of good and bad aspects with both. I'd say the reference implementation of Matrix (element) is pretty solid security-wise (good crypto, lots of eyes) compared to more amateur-ish XMPP clients, but on the other hand XMPP clients tend to play well with onion routing, while Element as a web client is quasi-unusable in the Tor Browser (lots of UI actions appear to be latency-sensitive) and using another javascript-enabled browser (than the Tor Browser) over tor defeats the whole purpose thanks to WebRTC leaks (and others).

I'd recommend you take a look at this FAQ. If something is not clear or missing, just ping us and we'll add relevant information :)

From the FAQ i linked:

While it is highly unlikely that a government would block the whole XMPP protocol (which is used for many things), it is entirely possible that they block specific servers, or block client connections to foreign servers, as is the case with many services going through China's Great Firewall for instance. (...) In the most extreme cases, it is possible for a network operator (or government order) to block the Jabber/XMPP network entirely. In this case, using censorship-circumvention mechanisms like Tor can help you stay in touch. However, please be aware that circumventing government censorship may be a criminal offense where you reside, and you may end up having trouble with your local authorities if they find out.

Both XMPP and Matrix can be reached via HTTPS so it becomes complicated for complete eradication. If state-level censorship is a concern of yours, Matrix is certainly more suited as a protocol as it has complex algorithms to resolve global state (consensus) in case some servers can't talk to one another.

Some other technologies like Briar are even more suited for this threat model, as it assumes all networks are compromised and/or unreachable. That is, it relies on gossip over lan-friendly medium (local wifi, USB keys..) with optional use of Tor onion services for reaching through the Internet without exposing so much metadata (beyond the fact you're using tor).

In Matrix client to server connections are just normal web-traffic, so that would require some deep packet inspection to block (pretty difficult but not impossible), however server federation uses a custom port and could probably blocked more easily.

XMPP by default uses custom ports and is quite easy to block by firewalls / routers, however it is possible to also use it via a normal web connection to circumvent that. That said, a lot of professional services use XMPP internally (emergency communication services, smart-meters etc.) and thus wide-spread blocking would be pretty disruptive for a lot of things. It is also possible to connect to and run a XMPP server entirely through TOR, which is pretty hard to block.

Neither is really great in that regard, but they are still harder to block then centralized commercial services (although Telegram tries to be "smart" by hosting their stuff on AWS etc, which also makes it hard to block as other stuff would be effected). Using XMPP through TOR should be relatively resilient though, but note that in some countries using TOR and other such services is illegal by itself AFAIK.