All of these discussions should really start out with some discussion about the type of soil you have as where I live (Western Oregon) there is very high concentrations of magnetite or something similar in the soil. (drop a magnet and pick it up and it will be covered with ferrous materials).
It seems common knowledge that in highly mineralized soils (mine) that a DD type coil is going to outperform a concentric of a similar size for depth with the opposite being true in sweet soils. That being said, each may be the right coil on a given day, at a given place, based on soil conditions, including moisture content and mineralization. Hence possibly the need for both of the coils you asked about....
Personally, I recently sold my 12" concentric but kept my Ultimate and a 950 concentric so I had a beach coil. The 12" was just too heavy for me and with its open construction, I couldn't skid it along the ground well without tufts of grass etc.hanging me up and the 950 with its more closed design just handled better and had a lighter feel to it when mounted. When I am in sweet enough soils or beach hunting I am not going to worry about if the coil is limiting my depth more than I am with either my tuning, technique or hearing......
I also have a 10" D2 coil and a 8x6SEF and love em both. For some reason, the 8x6 just seems to run smoother than the D2 or is at least quieter but I find no fault with either. I currently own 6 coils and figure each has its place at any given time and I use multiple coils instead of multiple machines to change up the game a little to get the most out of my PRO.
One last comment, Whites now sells the 8x6 and the Ultimate in competition with their own products which is generally an admission that the market has determined that those two coils outperform the factory equivalents. Not a great strategy in my opinion but nonetheless they seem to be acknowledging the performance gains of the Detech products.