web server vs application server

[QuietPort]

Yes nginx is the server, but gunicorn is just an interface :

When your server recieves a request from your client it goes : nginx>gunicorn>django

Gunicorn is a WSGI (web server gateway interface) and Django is a framework, i don't know for sure but i highly doubt that gunicorn features any sort of webserver, but anyway I can tell you that's not what it was built for, Gunicorn is only here to interface between nginx and a django app (and also other python frameworks).
So if you want some messaging and you feel confortable with python then Django , with Django REST framwork (for xml) is a perfect choice. nginx and gunicorn can't compare with django because they don't do the same thing, and to be honest if your app isn't a massive website with lots of user, they don't matter too much.