I use the double D myself and I can pinpoint an item really well with the wiggle technique. Just as you demonstrate, once the target disappears beyond the front edge of the coil, that is where I dig. I will probably will buy a concentric coil just to have as a backup and you never know, I may end up using it as my primary too. It'll be interesting to try out.
But one thing I've never understood is why folks get hung up on whether a target is say 3", 4", 5" or whatever the depth may be? I understand it is nice to see how deep your detector and coil will penetrate but it depends on the size of the object too. Plus if your detector states the target is 3" down, are you really going to dig just 3"? You'll keep digging until you find the target. That's just me though.