I think everyone assumed when these units came out that the 2 were identical, other than the Road Tech version having all the Harley Dealerships pre-installed. Unfortunately that's not true, I have discovered another very important difference.
Harley recently came out with a Navigation Interface Module or NIM. It's designed to cut audio on your radio when directions are given from the 660/665. I installed on on my bike and could not get it to work. Got a buddies Road Tech (I have a Zumo) and it worked fine. Further, my Zumo would not work on his bike with this module. The assumption now being made is that the Road Tech version of the Zumo has some kind of pre-instruction pulse that turns on the NIM module to allow it to work. The Zumo doesn't.
Steve Garmin Nuvi 750, 3760LTM and Zumo 660. Formerly Nuvi 350 and Axim x51v with Mapopolis and bluetooth receiver.