Normally those two dont go together...... small targets and depth. On VLFs the small coil is better for small targets but the larger coil gets more depth. However..... as to every thing there are exceptions.... based on your settings and trash. High sensitivity can reduce depth in a trashy area because normally it hits deeper trash causing more coil shut down. Now on a PI..... its different the small and large coils get the same depth because of design.
Your question depends on where and how you will be using the detector. Do you want to find small targets on a huge beach, or cover more ground to find normal coin size targets? Which coil works better is one of those Ford or Chevy type questions. Some will say their 8 inch is the best, I use the 10.5 to cover as much gound as possible in an alotted amount of time.
10.5" for depth.....8" for smaller targets.
I use a 10.5" coil 95% of the time both in and out of the water.......one of my water scoop have 5mm holes just for the purpose of retaining small targets detectable with the 10.5" coil.
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ivanll