Have a look at : http://www.softfinity.com/blog/the-ba...

A lot of the choice really comes down to what you are wanting from your framework choices.

Knockout is lightweight and fast and is becoming almost a first class citizen in the dnn space
it also has a wealth of third party support and plays very well with jquery.

Backbone is popular with the MVC community as its goal is to replicate that experience clientside.
Ember and Angular are bigger more complex systems - with lofty goals but fairly deep learning curves.

As they are all script engines - not exactly sure what you mean by true HTML5 extensions - beyond the fact that they all work with HTML5 applications.
With regard to "taking advantage of built in services(dnn)" again not sure what you are thinking here
- all the libraries consume data from webapi services in some way
- how they consume that data does vary - some do native databinding for example.

Westa