If I didn't have a Lab radar I would buy the Garmin over a Magneto speed mostly to not have something attached to the barrel.
While empirical data can be very meaningful as to establishing true bullet velocity and consequent associated data, as previously stated there is no standard to measure against or verify the accuracy of a given instrument. In real world application, and I hate to use the term, it is close enough. It can be more closely verified through bullet drop measurements in a controlled environment but to what end? I am not aware of a truly controlled environment where one could shoot 600 yards for precise bullet drop and if there were one would have to remove the ever present human element inducing errors.
In practical application, one could contact some folks with a variety of instruments which could measure the same shot from the same rifle ie. a magneto speed, Lab Radar and Garmin (or two) all surrounding the rifle when fired and compare results. The folks at Applied Ballistics a great white paper on their findings and what they use, unfortunately they don't include doppler chronographs unless there's something I missed