That all depends. Personally I prefer a smaller coil if the shore is littered with lead and other trash. It forces me to slow down, and really separate the targets. A smaller coil is nice to work around pilings and other structures where the large coils has difficulty maneuvering around.
Either one would work, but the real question is what is the bottom composed of. Targets in sand will work there way down by vibration of the water where targets in mud are only covered by sediment. The recovery tool is the question mark. Frank
I dont really see how 2.5" makes much difference, but I imagine a 10.5" would be more suited to actually diving. As you would want to cover area that has little trash reached by going down with a tank.