Dan,I'll be honest here....just MY experience with the 3 Minelab coils. I have used the 6" coil in places I thought might be a bit too thick with targets for the larger 2 coils and have NOT found anything the stock one wouldn't. These areas may have 5-10 signals in a 4 foot sweep,maybe more. Same exact settings except for higher sensitivity as usual with the 6" coil. That's ME. I could be doing something wrong, no doubt. But I just haven't seen it yet...AT ALL. Not ONE recovered coin the stock one didn't already see and I dug it before.
Don't take this as a turnoff,because I really like using the little coil,it's so easy to swing around and it will see the 6-8" coins with high sensitivity and if you're paying attention. In my areas I need depth to get to anything old,except in a few select places. That makes the 6" coil my absolute LAST resort.
Now, hunting in shrubbery,in tight spots,under porches....no contest,small coil all the way.