For what its worth here's what I would recommend:
Get a minnie ball, a general service eagle button, a small piece of iron (maybe a small buckle or nail), and a misc. small brass object (or whatever you have to represent the targets you're after).
Lay these objects on top of the ground (with exception to the iron object) so that you can check each object individually without one interferring with the other.
Set up your machine just as if you were going to hunt (GB, trac, etc.).
Check each object and take note of the audio response and distance from the target.
Then put the iron object within close promiximity of each of the other objects and run the test again. Then switch coils and start over.
This will illustrate your target response compared to your soil conditions.
With the DD coil it will also illustrate target separation in those conditions with iron present.
Results:
The minnie ball should ring in better with the 7.5 (lower conductivity).
The brass should ring in better with the 18.75 (higher conductivity).
You will see better target separation with the DD coil vs. the concentric.
If your soil is highly mineralized then the DD is the way to go.
If the soil is moderate to low then you should be o.k. with the lower frequency concentric coil accept where greater target separation is necessary.
Good luck!
