Selecting an MVC Framework (Javascript - Front End) - Backbone, Angular, Ember

I've spent the last couple days researching different arguments for and against different MVC frameworks but what's become very apparent is that there's absolutely no correct answer to this question as it's heavily dependent on a number of different factors specific to the underlying project.

The advice I am repeatedly coming across is select a framework on the following points: Ease of use, productivity, testability, community, documentation

While that's all well and good advice, I still think that there are a few more things to consider specific to the actual code-base you'll be working on.

That being said, hypothetically if you were to arrive at work one day and asked to select the "best" Javascript MVC framework for your code base to handle the entire front end of an existing product (keeping in mind that there is a current codebase and you'd want the framework to be able to handle the "transition" period as you shifted to the new framework over time), what are the factors/questions you believe would be necessary to consider to make a well informed decision? These can be factors/considerations/questions about the MVC frameworks themselves AND/OR the current code base / technologies current in use (ie. Ruby backend, other libraries currently in use, product functionality, etc).

Scalability and performance are extremely important as this product would have a fairly large volume of users.

Just due to stability and maturity of certain frameworks alone I've factored down realistic choices to Angular, Backbone + Marionette and Ember (although this one is still up in the air). Each one is a different beast by its own right and functions very differently, hence the need for my original question on what factors and questions would you ask to figure out which is the "ideal" (I use that word very loosely) choice?