I have been wondering myself, I really like the small coils myself. I have a 6x10 DD eclipse on mine and I "feel" that it does as good a job as a small coil. I had a 5.75 on my Vaquero and never really took it off. I don't see any loss in separation with the DD coil. The concentric coil should be better all around in normal soil, but.....
I am hauling out many coins that are on edge with the 6x10. I always felt that the DD coils had a weakness in this respect and air tests confirmed this for me with this unit. I am however very impressed with the number of on edge coins I am pulling out of the ground with this coil. If the DD coil offers better performance in nasty soil and does not seem to have an apreciable loss in normal soil and will hit the small targets and coins on edge.....? The one more extra benefit is that you cover more ground per sweep than you would with the concentric, since you effectively have a 9" sweep field with maybe 2" inches overlap.
I would really like some one who has both types of coils (DD and concentric) do a head to head comparison on:
1. on edge coins in the ground
2. solidly detecting, locking on to a piece of small gold
3. air test on max depth on a clad dime
4. VDI/TID accuracy/comparison at depths on same dime
The never ending search for the perfect coil/machine combo goes on! I love it.