Neil: Regardless of the coil used you are best off looking for fresh drops on dry beach sand. Best places to look are entrances, parking, above the high tide line where people put down their towels, beach concessions - anywhere money is handled.
Stay away from the wet sand or in the water as the mineralization will cause your detector to false alarm.
You are currently using a concentric coil which has a cone shaped pattern. It detects deeper and is easier to pin point as the target will be in the center of the smaller coil. It requires that you overlap 50% to 75% each sweep as it only covers a small area deep.
In contrast the DD coil has a narrow long pattern at the bottom, like a cleaver. As a result overlapping is not as important. However, there is some loss of maximum depth and it is more difficult to pin point (hint - use a wide scoop). It also handles mineralization better as you only see the narrow strip under the coil verses the entire cone seen by a concentric coil.
When I started detecting with the bounty hunter pioneer 505, also a low end detector, I had a lot of fun experimenting with coils, the trick being to purchase extra lower shafts so that you can rapidly change search coils. I had a lot of fun with it, and still use it to scout new beaches and hunt on dry sand as it is light.
You will later need to upgrade to a higher end multi frequency VLF or a Pulse Induction detector to hunt wet sand and in the water
I recommend going the multifrequency route with discrimination so that you do not have to dig everything. You do not want to use a PI where there is a lot of ferrous (iron) junk unless you like to dig, dig, dig.