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).

One of the better comparisons out there: https://www.freie-messenger.de/en/systemvergleich/xmpp-matri...