Another advantage of XMPP is that it can do encrypted calls, where as Deltachat has no call functionality, it only offers video or audio messages that can be uploaded to the encrypted chat.
The QR code connection thing, encryption by-default combined with a reliable universal app that works on all platforms, and email being difficult for state-actors to block are nice features of Deltachat, though.
It seems both have their place at the moment. It'd be nice if XMPP could eventually adopt the good parts of Deltachat.