Sorry I didn't know that tor messenger is just another xmpp client software with otr. I thought it using another protocol. Thats why there were strange talks about jabber transport I think. BTW Psi is xmpp-only client.
Tor Messager advantages are:
1) tor-integrated - whonix is already tor-integrated, so count out
2) "to be secure by default". Privacy friendly. May be. But it is small and yong project. Beta. In the simplest case, bugs. It's not even included in the repository. Where is TM/instantbird in-depth privacy/security review? On the words?
But on the other side: limited functionality and adjustability. For my Whonix configuration (vpn over tor) it basicly doesn't work at all (may be there is internal tor check or some stuff like that, didn't digging. "No proxy" setting like for TBB doesn't work).
So imho TM is like "bundle" for easily tor messaging. No need to configure it - perfectly for novice user.
Also I think pre-configuration and sanboxing (apparmor mb) are the best solution for "not privacy oriented" software like Psi etc.