Sencha touch gives you a lot of pre-made widgets, but its a lot of work if you want to customize them, or create something "off the beaten path". You end up creating a custom component and digging through lots of layers.
Backbone is more lower-level access and not as "packaged". If you need mobile UI components you can use something like jqtouch - also written by david kaneda, before he went to sencha. not as fully featured, but the basics are there.
