A major advantage the Garmin's have is the ability to plan your route on a desktop computer and load it into your GPS. Or plan a trip on Google Maps and upload the route to your GPS. As far as I can tell, any Garmin will allow you to do this. My brother has a very expensive Zumo. After looking at features and prices, I bought a nuvi 550. Waterproof to the same standard as the Zumo models. Garmin rates it as "motorcycle friendly", just like the Zumo's. Price is only $299 list, street price $249. It does not have bluetooth, XM radio, or an MP3 player. Since I do not want those features built-in to my GPS, I went for the lower price. The nuvi 550 is also a "multimode" GPS. It has a mode for walking (on streets), hiking (off-road), driving, and boating along with an 8 hour battery life and a user replaceable Li-ion battery. It has the track-back feature that will allow you to reverse your course turn-by-turn, which is particularly handy if you ride a GS bike off-road. You can take the trails that are not even on the map, and when you are ready to get back to the main road, the track-back feature will take you back exactly the way you came. Zumo's have this feature too, I believe, but I have never seen it on other brands.
I bought a lifetime subscription to the map updates last year when they reduced the price to $89. A single update is $49, and when the lifetime dropped below the price of 2 updates, it seemed like a no-brainer. They come out with updates about 4 times a year. I don't think the roads change that fast, but the points of interest definitely do. I have a Magellan I use in the car, and I have navigated to more than one boarded up restaurant. The other thing you get with the updates is updated software for the GPS device, which means better performance every time you update.
Another nice feature of the Garmin models is the ability to enter longitude and latitude coordinates directly. I can't do that with my Magellan. I have used that feature more than once to navigate to a Rally out in the middle of nowhere that publishes the GPS coordinates (invariably in Garmin format). If you are meeting someone, and you both have a GPS, the first one to arrive at the meeting place can read their location from their GPS and send a text message with the coordinates.
You can also save your track for a long ride to files which can be downloaded from your GPS later and displayed on your Garmin desktop software, or even in Google Maps.