Both XMPP (via OMEMO) & Matrix use libsignal for double-rachet encryption—so they have the same encryption properties. The biggest practical differences for the average user in my opinion is XMPP has a separate concept for DMs (not a 2-user room with encryption like Matrix), XMPP allows encryption to be both enabled then later disabled, & Matrix offers better resilience as messages & attachments get synced to all servers a room (which has a massive downside of resources, storage sizes, & moderation; if a server goes offline, you still have a history of the chat but if someone shares something explicit, such as CP, it will propagate thru the network & there is no way to delete it across nodes).
Lots of open source projects have matrix servers and not XMPP servers. Some bridges don't have XMPP equivalents (and some bridges don't have Matrix equivalents either).
XMPP also does E2EE of course, though I've found it to be a worse experience on most clients compared to Matrix.
