Erlang is simpler, there are less syntactic constructs to learn, has more information and learning resources available probably due to its age (30+ years),maybe more tools and libraries, that also means more example to look at. It is also the default language the VM is written for. So you can track and follow the latest changes as they are released.

However if you come from Rails or want to do mostly web development, then Phoenix is a nice framework, so maybe a good place to start. Also Elixir has a very friendlier community for newcomers perhaps, kudos to Jose and team there, they went above and beyond to make that happen.

All in all, there is a huge overlap. Some of the harder or more interesting concepts - functional programming aspect, using light-weight processes with isolated heaps for concurrency, performance evaluation and debugging will be the same. It is not like if you pick Erlang or Elixir you'll be wasting time and going in a separate direction, they share a lot of common concepts, and could transition easier if you need to between them in the future.