Chris,
Having owned a Whites M6 since 2007 I learned it very well, as I had been on the other forum posting my finds.
I have owned a Fisher, an old Bounty Hunter ( pre-First Texas ownership) 4 other Whites and a 1980's Garrett Master Hunter. I didn't like the Garrett because of every signal had the Bell Tone. It wasn't just a bing sound--it was binnnngggggg. I sold it after a few months. It was a great detector--just didn't like the sound.
The AT Pro, which I still am learning, is being used in areas that I had searched with both a Whites M6 and Whites IDX many times before. On my last hunt using the AT Pro I searched a 2:blackeye:20 foot section that was gridded with my M6 AND IDX at least 10 times using all the coils I had ( 3 coils for each detector) and getting all the items from the ground. The ground was clean--at least the detectors didn't find any other signals at all.
I used the AT Pro there yesterday, and dug up some deep clad, 2 wheats and a Barber quarter that the other 2 detectors missed in the section of ground, and I had dug coins at 9 inches with the M6 and 7 inches with the IDX, using the stock coils on them. So the depth wasn't a problem with those detectors.....as they did get good depth on coins.
Garrett AT Pro vs M6
3 tones M6 7 tones
Choice of Manual or automatic ground balance M6 1 way to ground balance
Waterproof to 10 feet M6 not waterproof
Can tell a bottle cap from a quarter signal M6 some caps read the same numbers as a quarter
Weighs just a little over 3 lbs. but feels coil heavy M6 heavier but is more balanced when swinging
Notchable in or out on anything from iron up to 1 dollar M6 no notch feature
Excellent separation and depth from a 8 1/2 X 11 inch DD coil M6 stock concentric coil good depth and separation
When I tested the AT Pro the local dealer had brought on a group hunt, within 5 minutes I dug a clad dime at 6 inches in an area with lots of bottlecaps........and I had just scanned where the dime was minutes before with my M6 and it didn't see the dime there.
Both the M6 and the AT Pro are very good detectors they do find coins very well. It is true Garrett had some issues with the coils, but so did Whites when they 1st came out with the 10 DD coil. Both companies fixed those problems.
I hope this helps out.