Per the other comments, if you learn Elixir for professional reasons, you're very likely to at least get to the point you can read Erlang, even if you don't write it fluently. And if someone is hired on who knows Erlang but doesn't know Elixir, I doubt that's going to prove a huge impediment, especially if they have any other scripting language experience. I'd consider the question of Elixir/Erlang to be more a "both-and" sort of thing than an "either-or".