I think Jeremy has a good answer regarding the different scopes of backbone and sproutcore http://documentcloud.github.com/backbone/#FAQ-rias

One aim of brunch is to provide more structure and good documentation about useful practices and patterns of backbone apps. I think while the stack is much simpler compared to something like sproutcore and cappuccino there are many (even rather complex) apps that can be built with ease.

It basically boils down to your preferences. I personally love html & css and prefer to have tight control over the look and feel of my application. I prefer easy to use bottom-up tools like flask and backbone over 'more sophisticated' frameworks like rails, django, symphony, sproutcore et al.