The Zumo 550 is pricey, but designed specifically for motorcycles. My wife got me one this past spring, and I love it!! Like Wecroft said, the 550 lets you set it for all kinds of different things. Well worth the money imo.
The Zummo is the best for a motorcycle and it also comes with a car mount so you can kill two birds with one stone. With the mapping software that the Zummo comes with you can pre set routes and load them into the zummo. As someone else said you can use the auto route and avoidences to stay off main roads. Using the mapping SW you can adjust just how much off the main road you want however it will route you on to some roads that may or may not be paved if you take it as far away from main roads as it will go.
I have had my Zummo 550 for a year and LOVE IT, I use it all the time to set up day trips and routes the mapping SW helps me to find roads that I would have never thought of taking in the past.