I find DD vs Concentric choice more a matter of target separation.......
I find the DD coils more useful in 'trashy' areas. It doesn't look at as big of a chunk of soil, so its easier to separate the targets.
As far as depth, yeah the soil mineralization plays a factor. Highly mineralized, the DD reaches a little deeper.
Low/neutral mineralization and the concentric reaches a hair deeper, but not much, only an inch or so.
When explain the difference to people, I tell that that for a concentric coil, image a ball cut in half. Place the half
of the ball under the coil, flat side against the coil. The half of ball now represents the shape of the concentrics coils
magnetic field under the soil.
For the double D coil...image 2 plates. Lay one on top of the other facing down. Then cut them in half. Now place that
half, standing on edge, with the flat part against the coil. That is the general shape of a DD coils magnetic field under the soil.