I spoke with a Garmin rep a few months ago about the differences between the Nuvi and the Zumo. Clearly, if they are almost the same, it's a no brainer to buy the cheaper (and still waterproof) unit.
He said, they're much the same - except:
The Nuvi does not have screens designed for use by gloved fingers on a vibrating vehicle like a motorcycle. The Zumo has some input screens in two versions - for motorcycle (larger icons, keys) and for car use. It can tell whether it's in the car or on the bike by what mount it's on, and switches automatically.
The Nuvi has a less bright screen than the Zumo. The Zumo screen is specially designed to be read in bright daylight. The Nuvi screen assumes you are in the reduced direct sunlight of a car or truck. This also accounts for the difference in battery life, I expect.
So, don't just make the guess that Garmin is just trying to gouge you by selling the Zumo for $300 more than the Nuvi.
That said, if you don't mind taking your glove off to input info, or having a substantial sunshade on your moto unit, the Nuvi is a bargain.
They just aren't the same device for the same main purpose.
I made the economic choice to just buy one unit (Zumo) and use it in the car and on the bike. The 550 comes with both mounts, and it takes 20 seconds to remove and reinstall it.