G'day
One thing to remember about dd coils is that they are designed to be swung from side to side, and the will also handle the hotter ground better than a mono coil, but if you are using a coil on ground where there is a lot of vegetation and you are pushing the coil forwards under shrubs and so on you will miss targets as the tip and the tail of the dd coil is less sensitive, a mono coil has the same sensitivity all around the perimeter of the coil so if there is anything hiding under the shrubs you will hear it.
A smaller coil is always better in areas of tight vegetation but of course will not get the same depth as a bigger coil, and the bigger coil is better for more open areas where you need to cover more ground and you will also have a depth advantage as well, once you are on to some targets with the small coil you should always as much as you can get a bigger coil over the area as well, sometimes you might have to remove some deadfall to do it but it can be worth the effort.
Mono coils generally will go deeper than dd coils but if the ground is too hot for the coil then the noises it will produce will most likely drown any iffy target signals anyway, on the Gp3000 you cant really do much about that, later detectors such as the Gpx4500 will run mono coils on the hottest ground in the enhanced setting, a great advantage in some areas, I once worked a spot that made my Gp Extreme impossible to work with a mono coil, although we got many oz's of gold off that spot with dd coils, I was blown away when a few years later when a mate and I returned to the area with our Gpx4500's and pulled about 14 oz's more out of the same area, some of the pieces were very shallow but were missed previously.
If the ground allows its always worth while to try a mono coil anyway, nothing to lose but everything to gain.
erfect:
cheers
stayyerAU