Are you looking for depth? If so the larger the coil the better. Mono coils will punch deeper than DD, but there is no discrimination with them. Nexus have brought out a concentric 21" coil for the GPX, its like two coils in one. The outer coil ring acts like a mono when switched to deep mode. When switched to hot ground both the outer and inner coils come into play to enable discrimination. This coil has two cables coming from it into a switch box which in turn connects to the GPX coil socket. This switch box has two settings, 'hot ground' and 'deep mode'.
I don't have this coil yet but I do have a collection of coils Mono, Anti Interference and DD. Including Minelab, Coiltek, Nuggetfinder and Deep tech.