if there is a lot of trash, use the small coil...
if the targets are far in between, use the large coil...
i have one detector with a large coil...if i am in a manicured lawn, i use that locator to find the targets, i mark them , usually around 30,
with some homemade markers ,
then i use the other detector with the small coil to to pinpoint and retrieve the metal objects.
good luck ....lou.