I've always been a fan of the Defelsko line of PTGs but they are on the spendy side.
After working with the people at Detailometer, I've come to trust and respect their engineering. Besides making the Detailometer, (which I have one and use all the time), I also have their Coating Thickness Gauge.
I picked up this one 9 years ago and have been happy with it. I use a version of Mike's Vehicle Inspection Form and take before & after reading on a dozen places on the car. I mostly use it for a 'go/no go' indicator. I have had a few cars that I could only hand polish(panels or whole car) based on the readings. ERAY Mil Thickness Gauge