In response to the question about the 15x12's sensitivity to smaller targets than the 10". I would say that's what it looks like. I've at least dug smaller targets at deeper depths with it on land than I have with the 10". For instance, shoe lace holes at 8 or 9" deep or even a small piece of aluminum about double the size of a pea at something near 11" deep. I can't remember digging those small of targets that deep with the 10", but that's not to say it won't also get those that deep. The 12x10 is said to get a half inch deeper than the 15x12 on a cut quarter penny, which from what I understand is even smaller than a half dime. Both coils get deeper on all coins than the 10", but from memory I think the 10" matched the 15x12 on the cut quarter penny. I've dug earing backings with the 15x12 that I couldn't believe how hard and stable the GT hit on.
Here's a quick anology of how I look at the coil size debate I touched on in the above messages. Look at the coil as a bucket and a body of water as the soil. Let's make believe there is a silver dime 11" deep floating in the water. The 10" coil (or bucket in this case) is able to sink down and scoop the dime up. The bucket might be full of water but the handle (in this case the machine's processor) is strong enough to hold the weight of the bucket going that deep and being that full of water. Now think of trying to use a bucket 12" in diameter to scoop up the coin. In order to go that deep the bucket is filling so full of water that it's handle (the machine's processor) can no longer support the weight, meaning the machine can no longer seperate the target signal from the soil due to the amount of ground it's seeing. Then imagine the sensitivity dial as a way to slightly adjust the amount of water the bucket will hold, or in reality how hot the coil is running and so how much ground it is taking in. There's a fine line there were the bigger bucket's capacity can be adjusted via sensitivity to reach deeper than the 10" bucket, but that all depends on how dense and heavy the water is. With higher mineralization in the soil the machine may no longer be able to process the ground load and recognize the coin. Adjusting sensitivity to even a much lower setting should probably still get deeper with that coil than a smaller one. Where that line is crossed from one to the other is a grey area. Sometimes a bigger hammer won't as easily hit the nail, but other times it will get the job done faster, or deeper in this case.
I would suspect that depending on where you live and the amount of minerals present the 12x10 might be deeper than the 15x12, or the 10 or even 8" might be deeper than all of them. With Crazyman's remarks about no higher than 3PM being what he can run the 10" coil at most of his sites, I'd suspect he's got even higher mineralization than me, because even using my method of calibration (which ends up being much lower than setting sensitivity via the normal method), I'm still higher with my 10" than he is. If I was to use my calibration method at his sites I'd probably be even lower than 3PM. That might work best for me, or his method might work best just due to that type of soil. All I know is the buried dime test puts me much lower than it would if I was just sweeping the coil around and adjusting it until the threshold was very stable.
For most of my experience with the 15x12 I've been maxing out sensitivity to whatever was on the edge of stability. That often turned out to be full sensitivity and this coil was still very stable, even much higher at sites than the 10" would run at. After a few frustrating hunts at known deep coin spots and coming up empty, I began to suspect that I might be costing myself depth by maxing it out even if the machine seemed to be running smooth as silk. Also, after digging a few coins deeper in Auto with this coil than I had dug any in manual I really became suspicous of my normal sensitivity setting methods that have worked on just about every machine I've ever owned. In fact, the only machine which also lost depth at a seeming stable high sensitivity setting were my Explorers. Once I began playing around with sensitivity on buried known targets it quickly showed me that what's best for best depth is far lower than what you would normally think. As it turns out this method of calibration also ends up being the best setting for my medium sweep speed while hunting for the next target. Even if it's not it doesn't matter to me. I only look for any change in the threshold, even a null. Once found I do the Sovereign wiggle over it. That's when I want the machine calibrated to provide me maximum depth for best target ID/audio. But, at least in my soil, even a slower sweep speed at a higher sensitivity setting doesn't hit on the targets as well for me, but again that could be soil, coil, or personal preference.
Play with both methods. Try calibrating the machine my way doing the wiggle. Then sweep over the target at your normal sweep speed and notice how it responds. Then adjust the sensitivity up and down from there with your normal sweep speed and see if you get a better hit calibrating the machine to that rather than the wiggle. You may find his method works for you where you live, or you may find that mine works. Only way to tell is to try and go from there. What I found most surprising was how what you would consider the normal random "null" as you hunt could in fact be a coin. Something much higher or lower in sensitivity will produce a null, or a broken/iron response at the very least. You would at least figure that too high of a sensitivity setting would produce any kind of signal and not just a null, but it does. That's when I realized something was going on here and that I was blinding the machine to the target by maxing out sensitivity.
Oh well, you can never say I lack in details...