I have used all three coils and my favorite is the 5x10 coil in 18.75 khz, i have very mineralized soil sometimes running sensitivity at 8 or 10 and i don't seem to loose depth, very common to dig 8" for a coin and loves gold. The 6" DD is good for coins but not gold, it won't hit on most gold rings.
The 5x10 used very slowly i thank is as good as the 6"DD in my area
Just a shout out for BART at Big Boy Hobbies, if you haven't called him and done business with him i encourage you to do so, he is a true friend and will go out of his way to help you get the right tools for detecting. Give him a call today!!