Hello Elmtree3 , welcome to the forum . :wow: you've had your DFX roughly as long as I mine :beerbuddy: . i prefer a double D coil over the standard one for a few reasons .
its a much better coil in the trash (you can isolate some targets in the trash better and hear just them) and mineralized ground .
I personally believe it hits harder on the deeper almost on the cusp "iffy" targets that the concentric one tells you its "garbage" .
the drawbacks to the coils ....
they take a little getting use to find the "sweet spot" on the coil, to pin point where the target is under the coil.
this is especially true when finding small iron like a bent nail which can sound pretty good depending how least amount of discrimination you set.
Now i have never used the D2 coil but i have many close friends who do (on v3i's , mxt's and on dfx"s) and they have all gone through a couple of coils each. Which is the reason i never gotten one , i settled for the detech SEF 10x12 "butterfly" . I can't complain about it , its has great coverage , is deep and has very good separation for its size but if I decide to get a second coil it would be the detech 6x8 "butterfly" for those trashier spots .
Dan