I have been solidly in the just use Elixir camp, but I'm debating that on some new projects. For one, if you want a library to be used in both, it's best to write it in Erlang. Then Elixir can just directly call the functions or you could build Elixir-specific wrappers of the Erlang functions. The other is that Erlang just has a bit of simplicity and rawness about it that I like, not to mention it is the OG language. I would miss piping and variable name rebinding though.

At the end of the day, I do think that Elixir has the better tooling ecosystem. Mix, Hex, the formatter, Credo, ExUnit, Livebook, etc. are really nice.