The less depth than the 12x10 was on a cut quarter penny, which is a TINY piece of silver about half the size of a half dime or less. Any smaller coil will get better depth on something that small, and the 15x12 only had a half inch less depth than the 12x10 on it. 5.5" is pretty darn good for any coil on something that small. Penny/dime sized targets and bigger the 15x12 most people say gets better depth than the 12x10, but both (again, most say) are deeper than the 10" Sovereign coil or the 11" Pro Coil. In fact, with all the digging I've done on the net about this coil most opinions are that it's the deepest large coil they've ever used on coin sized targets, even beating the WOT out.
Compared to the WOT the majority opinion on the 15x12 is that it's more stable, has better separation (almost like a 10" coil), allows higher sensitivity settings, doesn't false as much on iron, is more sensitive to tiny targets, provides better audio harmonics, pinpoints better, and so on. It's a winner. Remove the coil cover and it's lighter than the Tornado 10". I plan to spray bed liner on mine because without the coil cover it drops about 3.6oz and makes it lighter than my 10" Tornado with cover on. From what I've read the 18x15 is past the point of deminishing returns on penny/dime/ring sized targets but still an excellent coil for relic or cache hunting. I wanted the deepest coil on the market for coins/rings, and after several months of research I'm confident the 15x12 is the one.
One other interesting note that I've read alot of remarks about is that people are surprised that they are digging not just deeper coins than they did with other coils, but they are also digging shallow coins that other coils somehow missed. Some of these were either on edge or mixed in with trash. Now, I'm not going to claim that even with it's excellent separation that it's separating trash better than say an 8" coil (some think it's close to a 10" and even a few think better than that), but I've heard many people indicate that maybe the field dynamics of these unique SEF designs are hitting those coins in a different way and so is better at unmasking them than typical DD or even oval & eclipse designs. When you think about it, many of these coins that typical DD designs can unmask have been found, but by all rights there should be a lot of other coins masked or on edge in a certain specific way that the SEF magnetic field has an easier time revealing.