The main diff b/w the 2 IMO
Backbone is more customizeable and suited for larger apps.
Backbone stays out of the DOM for the most part so it's a little bit more manageable long term.
KO does great w/ smaller multi-page apps that need a lot of complex forms etc, like you said.
The thing is, you can make Backbone have the 2 way binding style of Knockout with something like https://github.com/NYTimes/backbone.stickit, it just takes more setup. But, it also might be worth the time and effor to set up.