A DD coil (like the 8x11) is made for and operates better in "bad ground" - that's it. If you have bad ground you may get a bit more depth out of it VS other coil designs but will surely get smoother operation.
If you have avg. ground and want better depth and really good target separation you get a Concentric coil
if you want just plain old target separation at its best you get a small coil (ie. a 5x8) or 5in round, etc.
if you want good depth, target separation, and stability all in one - the ellipticals are good for that (if your machine has one available for purchase)
if you want the deepest seeking coil - LARGE DD's or S.E.F's are they way to go (but expect more than 1 target to be under the coil 1/2 the time) or in other words - target separation at its worst as the detector will be trying to decipher multiple targets at once a lot of times (best used in woods or in farm fields, etc.) where max depth is needed and targets are limited.
