Elixir definitely has more momentum at the moment (mostly due to an influx of Rails refugees IMHO), but as someone who came over to Elixir from Erlang rather than from Ruby I think you will find that it is not hard to switch back and forth between Elixir and Erlang once you get used to it. Elixir, if you can get past the Ruby -ish syntax, lifted a lot from Clojure as far as what is happening underneath the surface and you can call back and forth between Erlang and Elixir at eny time. This lets you do things like write large chunks of your app in Erlang and then glue everything together using Elixir/Phoenix to take advantage of the larger Elixir web-dev community. Phoenix itself is based upon a lower-level HTTP library written in Erlang.

Not perfect, but it does allow you to leverage the advantages of Elixir (e.g. Phoenix and the related web-dev community) while still letting you get most of your work done in Erlang.