The 15x12 just isn't as popular as the 12x10 on both the FBS and BBS units. It's more of a specialty coil for large open crop fields or huge beaches where coverage is of primary importance. I traded mine for a non-working 12x10 a few years back, because the 15x12 just didn't show me even as good of depth as my stock 10" Tornado in my soil. Only at the beaches did it for some reason get more depth than stock on coin/ring sized targets, and what I find odd about that is my beaches are mineralized just like my land sites.
The 15x12 is one very stable coil for it's size, but too much like my 12x10 in traits and I feel my 12x10 is deeper and separates better. Only reason I'm adding a 13" Ultimate to my line up is because it's as different from an SEF as it is from a conventional round DD, so I see a place for it on days I want to grid large sites even faster than the improvement of the 12x10 over stock in that respect. When I force myself to take off my 12x10 for land hunting, I don't want to replicate it in some respects and want something different. The loaner Ultimate I had was that, and I've got a line on a new one a friend was able to secure for me that should be arriving within a few weeks here.
The 12x10, finding one used is a very hard thing to do, just like it is for the 8 and 10" Tornados. That should speak volumes about how much people like these particular coils. When the SEFs first came out you'd see 12x10's and 15x12's being sold and bought used on Findmall as people sampled the wine, so to speak, but now that the initial rush of triers is over ones bought new these days seem to stay put with people who know what they are getting.
