I have used both extensively.
660 on my old GSa and Nav 4 on the K1600. As far as I am concerned they are the same unit. I never had full bmw mount with the 660 and so never missed it. The Nav 4 has an icon to find your nearest bmw dealer but you could easily programme in the locations if you wanted that feature.
What is important is that, certainly on the K1600, is that the music feature on the Nav4 is disabled. Probably since the bike has its own iPod link. I am not sure if that is the case on all Nav4 units.
Aside from that they are the same and in 60k miles I found the 660 more than sufficient for Navigation, Music (16gb micro sd card) camera alerts and Phone.
That is the route I would go down.