I bought one of these, and I like it a lot. With the Garmin screw, you need to use Garmin's teeny tiny special screw driver, and you also need the eyesight of a 10 year old to see the darn thing. At 47, I cannot see the Garmin screw w/o reading glasses. Also, they way I have mine installed, I do not have enough clearance between the Zumo and the windshield to use the screw driver.
The thumb screw is simple and easy to use. It ensures that the Garmin is solidly connected to the bike. I have no connection to the sellers. I am just a satisfied customer.