Garmin Mobile PC is the only official option if you want Garmin Nav in a carPC. To answer your subject question, from what I've seen the Nav on PC group is pretty small compared to PND/phone/aftermarket Nav group. This basically means if the nav company's main focus is the PND group the PC portion is usually behind in features.
However, if you are looking at building a CarPC for nav only then a carPC is probably not your best bet. If that is the case I would look at an aftermarket stereo that offers the nav software or features you prefer.
I have iGO8 which offers a lot of the features you desire but it was never officially released for the PC and map support dropped off in 2009. I know soon I will have to go back to Garmin Mobile PC or some other software since Nav-N-Go has no desire to release a PC version.