Hey mate
Without getting nailed by someone who has another idea, as a noobie you should run a mono and as long as you are in fine gold, enhanced, or sensitive smooth.
The mono coil allows for very good sensitivity and because the outer edge of the coil is the sensitive part of the coil, it allows for easier location of the target for pinpointing as you can push the coil back and forth to the target and locat it easier.
DD are used for really mineralized ground, and under power lines, the detection zone on the DD is through the centre and can be a bit hard to pinpoint a target for newbies and can be less sensitive to smaller gold.
Stick with the mono as youve got one of the best detectors for small and large gold and to suit a mono.
Once you get the hang of it and you then find some terrible ground you may want to try the DD