The only other language that has any commercial acceptance is Elixir, so really it boils down to those two.

Elixir has more syntax to learn than Erlang. You could learn just about everything there is to know about Erlang syntax in a half hour, with the exception of a few advanced features like list comprehensions.

Elixir's syntax is much more familiar to most developers. Most consider this a good thing; I'm biased because Erlang's unusual syntax helps me think differently. If I write something that looks like Ruby, I tend to think like Ruby.

Knowing Erlang can only help with Elixir, but it's not necessary. There are probably more jobs with Elixir these days, which is a good thing. I guess I'm not really helping, so I'll shut up.