i use the 9x12 the most seems pretty good for me, have the 10x14 just seems big and heavy also have the sniper good for trashy areas with alot of iron in it seems fine also haven't tried the dd coil.
I have the 4 inch coil, the standard one that comes with it and the 10X14 double D coil. The standard coil is great for every day use and the detector is finely tuned to it so it is the most accurate. The small coil is what I use when there is a lot of mixed signals in the ground. It maximizes the likelihood you'll see the individual items. Using it I've found large items (usually iron) a foot down. I wish Garrett would make a small (4X6?) double D coil, I'd buy it in a second. The DD coils give a smaller more defined search field. OK, the 10X14 DD will see a dime but I haven't found smaller items using it. I use it when I want to cover a lot of ground and I'm not being super particular. It is good for looking for deep large items too. It has issues however. It will give a false signal when two items are about 10-12 inches apart and you happen to scan across them. It will tell you that you have something half-way between the items. If you use the 10X14 DD and don't find something, scan around the hole looking for two items. It will also "edge detect" a large target. You will get two beeps one for each edge that swings past the object. Basically the coils are tools the same way that wood planes work with wood. They look similar to each other but each of them are better suited to the task they were made for. The coils will take some time to get used to but they are all good...
