Chuck I am just a end-user not a tect person, but I have used my Goldquest for several years on beaches of Penscola, Destin, Panama City, Bradenton, Sarsota, Marco Island, Key West, Miami, Hollywood, Ft. Lauderdale, Vero, Sebastian Inlet, Daytona, New Syrma, Jacksonville and many, many in Ga, SC, NC and UP. I live in my Motorhome and travel and detect.... My experience is you can say Florida Beaches. The beaches on the Pan Handle are the quieted to use the 11" coil in. Discussing 11" mono coil here. Bradenton has some type of minerals that cause a bit/slight of threshold rises when on the wet sand and in the water and you have to run in 15uS. to dig the deep ones. Vero is the worst...Black sand which is iron and silver oxide in layers. Most of the time below Jacksonville I just find it better to run in 15uS to be able to hear the deep ones.
Recenly I bought a 10" mono. I lost about one inch in depth and I lost on dia. of radiated pattern. It seems to be more stable in threshold tone (sees less acumilated area of ground). So far appears to be more stable in the water but my experiece is limited on that yet.
I Have no experience with the DD coil expect on my Excalibur 1000 but the several I have had conversation with and what I have read the best choice for Florida might be the DD coil because it is far superior on hot ground I am told. May lose one inch in depth over the 11" mono but if you get stability you can hear the deep ones...You can't find it and dig it if you are ignoring the slight threshold rises...
Hope this helps in you thinking if not send me an e-mail and I will try to give you more for your area information...Frank