While the 10.5 inch DD at 7.5 kHz will cover more area with each swath, in my moderately mineralized soil conditions, I prefer the stock 9-inch coils. I beleive I can detect just as deeply with the 9-inch 3 kHz and 9-inch 7.5 kHz as I can the 10.5-inch DD. Granted, it takes a few more swaths to cover an area, due to the surface area of each coil. And, I need to be more aware of overlapping each swath due to the DD vs Concentric design differences. But the lighter weight 9-inch coils make it an easy solution. I do use the 10.5 at 7.5 kHz if I get in a wide open area with smooth ground. It glides quite nicely! But overall, this old body prefers the concentrics for depth, balance and maneuverability. You will find others on here who prefer the DD. So, it is simply a matter of personal choice, if your soil mineralization doesnt' warrant a DD coil.
The 9-inch concentric coils are available in 3 khz, 7.5 kHz and 18.75 kHz. The 10.5-inch DD coils are available in 7.5 kHz and 18.75 kHz. The 5 X 10 Elliptical DD coil is available in 18.75 kHz. And the new 6-inch coils are available in a concentric at 7.5 khz and a DD at 18.75 kHz.
HH Randy