While AngularJS is a little light on OO class functionality, it has a lot of great features that enable very fast development. Some of these problems can be mitigated by pairing it with Closure, which it is intended to play well with, and are both Google frameworks. Backbone.js seems to have a little stronger foundation for applications, but is missing a lot of really important features (strong templating, 2-way data bindings, etc). There are a number of plugins for Backbone which fill in these gaps, but industry and community support for them is mixed.
After a couple days of research and debate, we’ve decided Angular shows the most promise. We’re going to give Angular a try on a smaller application before we commit on any big ones. Check out the AngularJS Tutorial, applications come together very quickly. The Twitter Bootstrap plugin is also worth a look.