This will depend on the size of the target that you are looking for because the size of the coil directly affects the size of targets that a gold detector can find. Smaller sized coils are best for small targets because they are more sensitive. Smaller coils are also great for locations with a lot of trash, where you are looking for smaller sized targets. Larger coils can read deeper into the ground and quickly cover areas. However, you will lose the sensitivity of a small coil and large coils tend to struggle with high mineralization.
Along with size of the coil, there are two different types of coils to choose from too. Concentric coils have the best discrimination and are perfect for locations with a lot of trash. Widescan coils don’t have as high a level of discrimination but do a lot better with locations that have high ground mineralization. The best type of coil will depend on the specific location that you are hunting for gold in and the environment of that location.