two new 'stock' search coils, the good old 8X9 Concentric and new 8X11 DD.
You didn't mention what kind of need you have for it, the site conditions, such as sparse or very trashy, or whether it is ferrous junk or non-ferrous trash you'll deal with. So, let me answer it from the standpoint I have for picking any search coil, and that is to be able to deal with dense brush, short field stubble and/or short stiff weeds, densely littered sites that abound with nails and other ferrous trash, and, if I want to hunt longer periods in such sites without fatigue, 'comfort.'
I have tried the new DD coil along with others in some tough side-by-side comparisons. Track down Cal Cobra on the Findmall Forums and ask him how his Tejón with an assortment of search coils performed on my challenging Nail Board Performance Test in a Nevada ghost town last month. Now, if you are hunting a relatively clean environment and don't need any Discrimination to deal with iron litter, that's another thing, but I put the 8X11 to the test and had the same results Cal Cobra had ... terrible.
He had the Tesoro 6X10 DD off a Lobo SuperTRAQ, a NEL Sharpshooter DD and the new 8X11 Tesoro DD. Setting the Discrimination to just barely reject the four iron nails, he compared all four coils. Failure to pass the NBPT which means all of those coils were poor choices for very littered sites. Was it the coil's size? No, it was the coils type, as they were all Double-D and those struggle when hunting in densely littered places.
He didn't have a small 6" Concentric like I had on my Tesoro's, but he did have the other 'stock' coil, the 8X9 Concentric. Also what some would consider to be a coil that is too big for such a challenge, but I was ready for the surprised look on his face when he mounted it to compare with all of the DD coils he had. It passed the NBPT with 100% impressive results.
So, it is really a matter of what you are looking for in a coil, and you didn't mention that.
I have compare it in some highly mineralized environments, too, where people think a DD ought to out-performance a Concentric coil. Nope, not when it was trashy, and just barely edged the 8X9 Concentric in some clean spots. Back in the very latter '70s and though the '80s there was a bit more noticeable difference in 'bad ground' performance, but search coils have been improved as well as the detector operating circuitry, and while I have some DD coils in my detector arsenal, my preference [size=small](when available)[/size] is still for a concentric coil, even in highly mineralized regions.
Let us know what the specific interests are that you have for the new DD coil vs. the former Concentric coil. I'm curious.
Monte