What sort of riding will you do?
If all of your riding is on the highway the Zumo series will suit you well and you will be completely satisfied.
If you will do any serious off road work and you want tthe ability to build a point to point route you will be happier with one of the Garmin chart plotters - 276, 478, etc.
I have a Zumo 550. I ride with a couple guys that have Zumos and a couplem with 276, 478's. We do a major off road trip every year. The 276/478 guys create th off road routes and if we need to re-route off road, they do it. Can be done on the Zumo, but it does not like point to point straight line routing.
Lots of discussion on ADVRider about GPS.