I used Knockout.js on a project before switching over to Backbone. My experience was that while Knockout is great for simple projects, it becomes a lot more complex than Backbone once things get complicated. All those data-binds in the HTML get really annoying and the application becomes difficult to maintain. I like my templates clean.

Backbone takes care of syncing your data with the server, provides you with an event system so you can subscribe to changes, gives you a sane convention for how to structure your views and then gets out of your way. It's also easy to extend, so you could write your own data-binding system if you really need it. A simple 1-way data-bind is quite easy to implement.

If you aren't writing a single page app and only need to make a primarily static page more interactive, Knockout is probably the right choice. If you are writing a single page app, you often need to sync your models and collections with the server, or if your UI is very complex, Backbone is probably better.