Sencha Touch - a full mobile JS library, along with widgets, animations and all sort of utilities for mobile html5 development.
Backbone - a lightweight library that provides all sorts of utilities for a JS app (structure, models, collections, key-value binding, templating...).
So I don't really see room for comparison here. Both are aimed at different things really. While some of their functionality may overlap, their aim is different. As for developing an app with phonegap and whether to pick Secnha Touch or Backbone - I'd say it depends on the app. I've done Phonegap apps with both - Sencha has a lot of things built in already. In some cases that means faster app development. Other times it might be a hindrance. Backbone gives you basic, non-mobile-specific utilities to build your app. Meaning, writing styles by hand, animations... To sum it up, it depends on the app and your personal preferences/coding style.
