There are 5 main guidelines to remember when choosing a coil for your detector:
Coil Size & Depth - A larger coil will typically find targets hidden deeper in the ground, but they can be less sensitive to smaller targets. A smaller coil is typically more sensitive to small targets, but does not go as deep as a larger coil.
Mineralization - A Monoloop coil will often have improved performance over a Double D coil but can be more difficult to ground balance and therefore possibly more noisy. A Double D coil is often more stable in heavily mineralized areas.
Terrain & Vegetation - A larger coil requires more open ground whereas a smaller coil can be easier to guide in tight bush or rough terrain. Obviously a smaller coil is also lighter weight than a larger coil.
Discrimination - In areas where discrimination is required, a Double D coil will provide a more accurate ID. In high trash concentrations, a smaller coil is preferable.
Search Pattern - A Double D coil will typically provide a 'blade-like' detection pattern from the toe to the heel of the coil, which gives excellent ground coverage. A Monoloop coil has a detection field that is typically cone shaped, which requires overlapping of sweeps to ensure thorough ground coverage.
Double D coils VS Monoloop coils
Double D -
They offer a full width for search pattern.
Produce less ground noise and require less tuning (with a manual balance detector)
They give off less false signals because of hot rocks.
In high mineralized ground they offer greater depth.
Less sensitive to outdoor interference.
Basically, they are good for high mineralized soils. 
Monoloop coils-
Good for low mineralized soils.
Usually, lighter in weight.
Choosing a optional coil for your detector still depends on the soil, the size of potential nuggets
and weight should be considered. Please call me and I will help you choose the right coil