I don't know enough about the internals of the languages to speak on that, but here's what has influenced my decision on choosing Elixir:
Community: smart people (who I respect) in the Ruby community are getting really excited about (and choosing) Elixir. People like José Valim, Dave Thomas, Bruce Tate, Chris McCord, etc.
BEAM and OTP: The Erlang VM and OTP have been battle-tested at Ericsson. It's known for having 9 9's of reliability and scaled WhatsApp to millions of simultaneous connections.
HEX: like Rubygems was for Ruby, Elixir/Erlang has a budding package management system via Hex. Hopefully this becomes the canonical source for libraries.
Phoenix: Rails popularized Ruby and became the framework of choice for quickly and pragmatically developing CRUD apps. Phoenix has Rails-like conventions in building an Elixir app and has built in support for websockets. This framework is built for today's technology.
Syntax: Coming from Ruby, the Elixir syntax is approachable and understandable. It's easy to jump into without having much prior knowledge.
Kinda subjective, but those are reasons why I'm excited and choosing Elixir to build future projects upon.