The Roadtech version of the Zumo 660/665 does come with a 2 year warranty. You do not have to buy it through your present dealer...you can get it through one of the 20% off HD dealers, if your dealer won't match the price. HD Roadtech was offering free lifetime map updates a while back, but I'm not sure if that's the case any longer. If you have warranty issues with the unit, you can deal with Garmin directly, or through your dealer. There have been some bugs with the Roadtech version, but I think they are sorted out now. I had to send mine back to Garmin for a replacement unit, but now all is well.
There's a couple of things to consider when deciding between the Roadtech and the Zumo. The first is the additional 1 year of warranty with the Roadtech. The second is whether you will want the turn prompts coming through the HK radio...there are two ways to accomplish this: First, you can add a stereo patch cable from the Zumo/Roadtech output to the Aux In port on the radio. That will give you the audible turn prompts WHEN YOU HAVE THE RADIO SET TO THE AUX POSITION, AND ONLY THEN. There are ways to add an Aux input to the back of the radio so you don't have the cable going into the front port, but that's another subject. Second, you can add the HD NIM module to the HK radio, and it will give voice prompts through the HK no matter the selection...i.e. CD/FM/AM, etc. Very important: The NIM module will only work with the Roadtech version of the Zumo 660/665, as the firmware is different, allowing the NIM to receive a pulse from the GPS to allow the voice prompts to interrupt the HK and come through the speakers on the bike, or the headset, if you have that feature on the bike. As mentioned, the choice between the 660/665 boils down to the 665 has it's own XM antenna that can get XM and Weather radar (both are seperate subscriptions). If you want XM, you can get the HD module for that, and the 660 would be the choice, if you do not want the weather radar function.
My opinion is that Garmin is the way to go, whichever version you choose. There are other good products out there, but Garmin leads the pack, IMO.
Mounting either of the units on the RG is a matter of personal preference. I like the chrome handlebar mount that came stock on my SERGU, but there is a mount made to bolt on top of the Tach/Speedo pod. The handlebar mount will keep all wiring inside the bars, for a cleaner look. However, I'm used to looking down and to the left a bit to see the GPS on the bike.
The NIM route costs more and is a bit more trouble, but is more "seamless" with regards to the final result. Plugging the stereo patch cord into the Aux port on the radio is the simple solution, if you keep in mind that you will have to put the radio selection to Aux when you want audible directions. Most of us did it that way for years. You can load MP3's on the Garmin unit and the tunes will play through the Aux port as well. They will not play any other way. You also cannot make Bluetooth calls on your cell phone without some other "dongles" added. You can pair the phone and see that a call is incoming, but if you want to make/receive calls through your headset, other steps are necessary, and they are not HD parts, but it can be done.
I would not trade my GPS on the bike for anything...looking for street/hiway signs is just one less thing distracting my attention from driving the bike and being aware of what's around me. The GPS tells me what to do without ever looking at it.