The benefit of using a DD coil is that if it is quieter and handles ground mineralization better.  It also facilitates the use of your discrimination feature on your SD2200 or GP Extreme.  The penetration profile of a DD coil is such that you do not need to overlap your swings as much. But it does not go as deep as a Mono Coil of equal size, it is not as sensitive to small gold as a mono coil, and it is harder to pinpoint with.   In most areas of placer gold in the Untied States, a mono coil will handle the mineralization without too much noise.

If are planning a trip to Australia then having a DD coil is a must.   Australia's ground is heavily mineralized and most monos are too noisy to work effectively.

The benefit of using a Mono coil is that it goes deeper than a DD coil of an equal size.  However, Mono coils can be somewhat noisier in heavily mineralized ground and they will not allow your machine to use it's discrimination abilities.  It is best used in the "all metal" mode.

A Mono coil is more sensitive to small gold and is easier to pinpoint with.  Most seasoned prospectors in the United States prefer mono coils over DD's.