It might be tough to answer in a general kind of way, without knowing specifics of the nature of the application, timeline, resources available (what types of devs, budget, etc.).

I've invested in Elixir/Phoenix, for a number of reasons. There are plenty of Googleable articles re E/P and scalability. Here's one: blog.carbonfive.com/2016/04/19/elixir-and-phoenix…