For me personally, one of the coolest features of Phoenix is that it natively supports concurrency. Communicating with a front-end via websockets is really easy to do with Phoenix, whereas in Rails it's basically unheard of. I know that Rails 5 is going to introduce ActionCable and that it's possible to write event-based apps with EventMachine, but these are ultimately poor solutions compared to what's capable in languages that take concurrency seriously (like Elixir).