Car home is just an alternate home screen that is supposed to give you quick access to launch the things Google thinks you are most likely to use in the car. The icons are bigger than the standard home screen which might make them easier to press while the phone is in the car dock.
The nav app is the same regardless of where you launch it from on the phone. And it is free (though you do have to have an active data plan since the nav app needs to talk to Google at least at the start of the trip).
