I'm going to rattle on a bit here concerning my conclusion on coils. When I first started detecting, I had the notion, as we all often do, that bigger or in this case deeper, is better. Turns out that notion isn't necessarily true. I've found that I get much more use out of my stock and smaller coils although I do have several large ones. - 11" and clean sweep for my Tesoros and the super 12 for my MXT
Granted, larger coils generally get a bit more depth, depending on what other coils you compare to. They most definitely cover more area and there's the rub. For most coin and jewelry hunting in parks, fairgrounds etc. they try to make sense of so many signals that they are often nearly useless as well as heavier and more tiring.
I have one park near me that using the MXT with the 9.5 STOCK coil you hit so much trash in some areas that the signal sounds like a machine gun firing and the display dances all over - detecting is totally frustrating. Put on the 5.3 or the 4X6 or similar and you'll begin picking goodies out of the trash one by one. In most other, moderate to little trash locations, I've found the standard coils to generally do a pretty good job on depth and coverage both.
With all this said, if one OFTEN hunts large, fairly trash free areas where you really need the coverage, or a location where there is quite likely to be goodies at the more extreme depths, then the large coils do have their place, worth having one in your arsenal. Also, even tho I've seen it mentioned in the different companies descriptions on the characteristics of larger coils, I haven't seen the loss of sensitivity on smaller targets they often mention. At least not down to dime size.
Bottom line is I'm a believer in "if you like it, use it" regardless of what others think or say, but for the most part stock and smaller coils will serve best hunt after hunt for most detectorists. My two bits.
HH
BB