Makes sense, there's no one answer to my question, because there are so many variables involved. So, it will take trying them out and of course it will depend on my needs. It may even make sense to do server side on some parts of the site and client side on others. I like the idea of being less constrained to my backend technology, which is what attracts me to JS MVC. I also like how concise Backbone.js is, it doesn't seem to try to do too much.
This is great. I've been considering moving to a client side MVC as opposed to relying on GAE. I started messing around with Backbone.js and I really like it. What are the downsides of client side MVC as opposed to server? Is it easy to bog down the client with the extra processing?