I'm a huge fan of Garrett's 5x10 scorcher coil. Although I've never recovered anything deeper than about 5-6 inches with it, it is the best combination of covering ground and target separation I've found. The sniper coil works great and will go 7-8 inches deep if you set the detector up right, but I get frustrated with covering so little ground. I suppose it would be ok if you were in an extremely trashy area with not a whole lot of ground to cover.
As far as double-D versus concentric, the best example I've seen was when I went to southern Colorado, where much of the ground is very mineralized. My imaging coil went nuts whenever I touched the ground - constant overload tone. The best depth I could get was 3 inches and the readings were very erratic regardless of how low I set the sensitivity. I replaced that coil with the 5x10 and it was smooth sailing - no overload, very stable and I found several targets at 5 inches.
Hope that helps.