Hi roadrunner,
The Harley Davidson GPS is a Garmin Quest that is rebadged and has the Harley dealership locations added to the software. It sells usually for more than the standard Quest but you can sometimes find it on sale.The dealer locations can be added to other Garmins too.
The Quest 2 is made by Garmin and is an improved version of the original Quest. Both Quest and Quest2 are portable and can be used while walking since they have bateries in addition to being powered on the bike or in your car.
The 2620 is a Garmin model that is a couple of years old and has newer versions out now such as the Garmin model 2720 or 2730 and they are getting ready to release the 2820 or 2830 soon. They do not have a battery system so they must be hooked up to a vehicle or other power source. Their advantage is that they are a plug and play model, you do not have to worry about memory chips of downloading the right maps to use it. You take it out of the box and turn it on and use it. They have touch screens that are larger than the Quest and the higher numbers can do more than just GPS such as XMradio,traffic updates and other stuff. Of course the more features the more they cost.
You may want to look at a site like
www.gpscity.com because it allows you to do a comarison side by side of each model.
I hope this helps you.