Prototype something in Ember.js first.

Is there anything you would change? If there is, you should remember that Ember.js is a very convention-over-configuration heavy framework.

Ember.js will do work for you, and if you agree with how it does that, it will be less boilerplate than using Backbone.js.

Backbone.js, on the other hand let's you structure code however you like.

Personal opinion: I prefer Backbone.js for my projects.