My two favorites currently are the 5 x 8 DD and the 8.5 x 11 DD. It's really just a matter of preference and where you tend to hunt. I do 90% land hunting in high trash areas.
For me, I prefer the DD's mostly because I hunt in high trash areas and have highly mineralized soil (i.e tends to be 90-93 on my AT Pro, a little lower on my AT Gold). I like the better target separation that the DD's have.
From the posts I have read, John does a lot of water hunting and indicated he likes the better target ID'ing and the pinpointing of the concentrics better.
As for the depth of the DD vs. the Concentrics, it is moslty a matter of the soil mineralization. In moderate to low mineralization the Concentrics will reach a little deeper. For example, when you compare the 8.5 x 11 DD vs. the 9 x 12 Concentric, you can gain an inch or maybe 2 deeper with the concentric, but only in low to moderate mineralized soil. In higher mineralized soil, say 80 or above, the DD reaches a little deeper than the concentric.
Because of the way the Concentrics work, they are a little more prone to target masking. Look at page 38 in the AT Max manual and you will see why. It is due to the shape of the magnetic field the coil generates under the coil/in the ground.
As for the soil mineralization. I am not trying to argue or refute what they told you, just going by the numbers stated in all 3 (Max, Pro, and Gold) of the detector manuals.
Note the lower the number, the lower the soil mineralization. The higher the number the higher the mineralization. It only goes from 0-99. So an 85-90 falls in the High ranges shown below.
Also remember to ALWAYS reset your detector when you change the coils. You reset it my holding the power button for 5 or more seconds when turning it on, you'll hear a double beep. You will have to re-select your preferred mode after a reset. You should also ground balance the detector every time you change the coil too. Matter of fact, you may need to ground balance it when changing detecting areas. Different soils can have different levels of mineralization.
Here are the numbers taken straight out of the Manual (page 24) for the AT MAX detector;
Typical Ground Balance Ranges:
80–99: Highly ferrous (magnetite, ferrous oxide minerals,
black sands, hot rocks, terra cotta)
60–80: Moderately mineralized soils (red clay, brown clay,
iron-bearing clay minerals, etc.)
20–60: Likely an iron object
0–20: Highly conductive, non-ferrous minerals such as
saltwater
Here are the numbers straight from the AT Pro Manual (page 32);
Typical Ground Balance Ranges:
80–99: Highly ferrous (magnetite, ferrous oxide minerals,
black sands, hot rocks, terra cotta)
60–80: Moderately mineralized soils (red clay, brown clay,
iron-bearing clay minerals, etc.)
20–60: Likely an iron object
0–20: Highly conductive, non-ferrous minerals such as
saltwater
Here are the numbers straight from the AT Gold Manual (page 18 );
Typical Ground Balance Ranges:
65–99: Highly ferrous (ferrous oxide minerals, black sands,
magnetite, hot rocks)
50–65: Highly ferrous, slightly conductive (hot rocks, terra
cotta)
30–50: Moderately to highly ferrous (red and iron-bearing
clays, most terra cotta, hot rocks)
0–30: Light to moderately mineralized (brown and yellow
clays, etc.)