Pure Ember.js or Rails+Ember.js?
We are going to implement our project with Ember; that's decided. But the burning question is should we do pure Ember.js or mix it up with Rails?
Consider using something lighter weight, like Sinatra that you can task with just providing a RESTful API to the backend. Unless you're wanting to sometimes render pages (which you can do with sinatra) to send to slow clients, I really don't see much of a use case for the full weight of rails.
Rails is too heavy, especially when you're using a front-end framework