The nuvi 500/550 also has a few features the Zumo line does not. It is a multi-mode GPS with modes for driving, walking (on-road), hiking (off-road with topo maps), and boating (when marine maps are added). To support the walking and hiking modes, they rate the battery life at 8 hours (more than twice the life of most).
Originally Posted by DRw2ge
One of my reasons to go with the nuvi was significantly lower cost. We have all experienced the progression of computer based electronic devices, and I am assuming the GPS units available in a few years are going to be faster with much larger data storage. I would guess there will be 3G or 4G cellular network integration and / or wi-fi for automatic map updates and web connectivity to add features limited only by imagination. If I spent $500 to $700 for a GPS, I would be reluctant to buy something newer for a long time. By spending ~$250 for a nuvi 550, I can justify buying a new unit with the new features much sooner.