It heavily depends on what kind of web application you are building. Is it a page-like application with very little dynamic elements (like an online catalog) or is it an application -like application (yes, that sounds stupid)?
This is a somewhat dated but still relevant example of using Cappuccino with Rails by the great Thomas Balhazar.
This shows you how to use SproutCore 2.0 with a Rails backend
And finally, this shows you how to use Backbone with a Rails backend.
So it comes down to deciding what kind of application you are building.