I looked the trailtech website to check out the voyager. Quick impressions after downloading and skimming through the user manual:
Pros
Designed for dirt bikes, with special features not available on standard handheld GPS such as odometer, engine temp.
Mounting is better suited for dirt bikes than standard handheld GPS, unit is very well protected from damage on most bikes and unlikely to fall off. If you have a steering damper, however, you would have to raise it up where it is less protected.
The screen is big, same size as Montana.
Cons
There is no way to load detailed maps (e.g. Canada Topo) into the unit. I like having the Canada Topo map, it shows a lot of information about the area, e.g. streams, roads, hills, lakes and so forth. This is very useful if you aren't simply following the GPS track of a pre-defined trail, e.g. trying to get back to the car by the shortest route possible if running low on fuel, or finding the easiest way back on a dirt road. You can create new routes without having ridden them for the Voyager using Google Earth, but this must of course be done before the ride.