SimpleX recently got the private routing feature which sends out messages over your trusted inbound SMP server. That way you won't ever connect to any servers except the one inbound relay your can set to a trusted one. 

If you assume with XMPP that both users are using Tor, than I would say if both users on a SimpleX chat are using Tor with random SMP servers, it's just as secure. 
No information can be gained by the SMP servers except that there are messages going out from one queue and that's it. 
SMP Servers only ever get to see one side of the communication, never the reply. Add to this the absence of user accounts, you cannot build a social graph and tracking by IP does not work if both sides use Tor.  

Interested in your thoughts, but SimpleX over Tor is a much lower barrier to entry than both sender and receiver having to anonymously host a XMPP server behind Tor. 
They also would have to run a new XMPP Server per person they contact so as to not link identities by using the same .onion for more than one conversion.