The answer is "it depends." What features do you want? MP3 player? Stereo? Weather & traffic? Internal battery? High sensitivity receiver? What's your budget?
I've owned or own all the 2xxx series StreetPilots a [large] number of Nuvis, and currently own a Nuvi 500, Zumo 550, and Zumo 665. In no particular order.
1. Automotive GPS prices have dropped sharply as have map costs. The smart phones with GPS have really eaten into that market. I expect this to be true for the motorcycle market as well. This means for basic navigation an automotive unit and a waterproof housing of some kind is the cheapest for basic navigation.
2. The top-of-the-line Zumos come with map disk to load on your computer, motorcycle and car mount, and AC wall adapter. They also allow routing through multiple via points. Whether you want to pay for all those features is something only you can decide. Those of you who don't think it's worth it? Check what used ones sell for.
3. The Zumo 660/665 has a larger screen and a much more convenient form factor than the 550 and earlier Zumos. It's touch screen only but that has worked well. It has stereo MP3 and bluetooth.
4. The Zumo 550 has hard buttons as well as touch screen and works well. It has bluetooth support and an MP3 player, the MP3 is not stereo through bluetooth.
5. The Nuvi 500 is waterproof, has a removable battery, and comes loaded with topo maps. But it doesn't have an external speaker jack or an MP3 player and doesn't come with an MC mount, though they are available as an aftermarket part. The Nuvi 550 has street maps.
6. Some people still like their 2xxx StreetPilots. They have some nice features and are pretty cheap but are large, don't have an internal battery, and don't have a high-sensitivity receiver.
I have a Droid phone and the GPS navigation works pretty well. And most of them also have an MP3 player and FM radio. But they aren't waterproof and I haven't found a really good motorcycle mount for one. Most of the guys I know that use them put them in the map pocket of their tank bag.
If you look at the pic I centered the image on Fenway Park.