If your bike has the GPS prep already, buy the BMW Navigator. Has a bunch of features that integrate with the bike, as well as rudimentary media player (via bluetooth), and works with you cell phone for calls, and some network features.. Can also buy relatively cheap mount to transfer from bike to bike when needed (I use mine on my KTM).
If you don’t have or don’t plan on getting the GPS prep for your bike (or isn’t available for your bike), there isn’t a good reason that I can think of to buy the BMW Nav V/VI over the similar Zumo line.
The Montana is a good unit as well, a bit smaller screen than the Nav/Zumo and no sort of media features, but most agree that it is a better GPS. Can load different maps, custom symbols and stuff that the Nav/Zumo cannot.