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 :)