The big reason to use something like Elixir, Erlang, or of course Node.js or other things of that ilk is if you're using a lot of web sockets, streaming, or that kind of thing. With Rails, a connection that stays open is (unless they've changed it recently) going to hog a whole Rails process, which is a lot of resources to tie up.

A secondary reason might be if you're doing pretty much everything in Javascript on the front end, a lighter server process is probably going to work out better in terms of resource usage. Myself, I'd probably still opt for Rails while developing, just because it's so fast to get something up and running with, and you're going to need to do stuff like authentication server side anyway.

If you want something that's vaguely familiar, you might have a look at Chicago Boss. It doesn't do as much as Rails, but it's fairly easy to get up and running, and the guy who built it is very friendly and helpful: http://www.chicagoboss.org/