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).