Phoenix is ludicrously quick to get from zero to realtime UI and interesting backend work such as polling, consuming updates, sharing state, collaboration.
Having done a bunch of Python I wouldn't discount Django but I would pick Phoenix almost every time at this point. The initial burst of Django doesn't seem like a long-term upside and Elixir/BEAM/Phoenix has a lot of long-term upside in my eyes.
Comparing DjangoPackages[0] and awesome-elixir[1], sure you could say "there's an elixir package for everything", but how battle-tested are they? How well-maintained are they?
If someone is trying to decide between Django and Phoenix, I am not sure how likely it is that the individual would have the necessary Elixir/BEAM experience to be confident in dropping-in erlang libraries.
[0] https://djangopackages.org/ [1] https://github.com/h4cc/awesome-elixir