Try both. It may be because I've been doing Erlang so long but I still prefer it and its tools (like rebar3 and Common Test). I find the syntax more consistent, less verbose and the lack of Elixir style macros to mean less confusing third party libraries.
But I do think Elixir has its strength in web applications. The macros make Phoenix and Ecto much simpler for building full web applications and if I were building a full fledged interactive web application I'd reach for those.
So mainly personal preference, you may find Elixir preferable, but all that to simply say it is not a "strict improvement".
Oh, and one none personal preference reason: it is nice to do libraries that don't benefit from being written in Elixir (like postgrex and jason get a lot of performance out of using macros so there isn't an argument to write those in Erlang) and aren't specific to Elixir (like a Plug adapter) in Erlang so they are more easily usable across BEAM languages, a list that continues to grow.
But I do think Elixir has its strength in web applications. The macros make Phoenix and Ecto much simpler for building full web applications and if I were building a full fledged interactive web application I'd reach for those.
So mainly personal preference, you may find Elixir preferable, but all that to simply say it is not a "strict improvement".
Oh, and one none personal preference reason: it is nice to do libraries that don't benefit from being written in Elixir (like postgrex and jason get a lot of performance out of using macros so there isn't an argument to write those in Erlang) and aren't specific to Elixir (like a Plug adapter) in Erlang so they are more easily usable across BEAM languages, a list that continues to grow.