I'll do my best Bryce...Honored to even be asked when we all know which way the wind blows on FBS coil opinions- meaning you are the one to ask on that.
So Swingnlow, being that I have soil that ranges from none, to mild, to high minerals, I guess I *might* be able to help just a tad in that one small respect, but as we all know no guarantees your soil acts like mine. One man's bad mineralization can be a very different animal to another.
Another dis-qualifier- Is that while I've owned 3 Explorers (soon to be #4), my experience with the 12x10 and Ultimate is on the Sovereign GT, and we all know coils can have very different results on different detectors they are made for. However, I will say that one of my hobbies is to read all I can on coils for the FBS and BBS machines across the web in some pretty obscure places, so I can at least say that in terms of the SEFs and the Ultimate, both coils appear to have very similar results on both the Sov/Excal and Explorer/Etrac machines they are differently made for. My testing, in fact, shows the Ultimate experiences I'm having pretty much mirror what Bryce said in his field reports. Same deal with the 12x10. But, that said, still take my "BBS opinion" with a healthy grain of salt and seek final word from those in your "tribe".
Now that's I've covered my behind with enough disclaimer to ward off most criticism, I'll say this about the soil thing *in my case*...The 12x10 runs at much higher/smoother/more stable levels of sensitivity in my soil than my stock coil. Best coil I ever used, and let's just say the stock coil ain't no slouch, so that's saying a lot. The 12x10 is deeper, separates more "effortlessly" left/right wise, and even makes target response more "crisp". It's not only able to run over bad grounds at higher sensitivity levels without as much instability, but it's also less prone to EMI and so can run at higher sensitivity for that reason too.
I was talking compared to my stock 10" coil (which is a much better coil than the stock 10" coil on the old Explorers I owned), but in comparison to the Ultimate in bad soil, the meat of your question, here is the gist on that- The Ultimate won't allow me to run at the same sensitivity level as the 12x10, at least at some of my worse soil sites, or it will null out more. And that's not due to EMI on those occasions, because the way to tell EMI versus ground matrix instability issues is EMI chatter when the coil is held still, versus nulling or instability only when it's moved (meaning ground matrix issue).
That said, the Ultimate is a very stable coil for one of it's size, but is it's lower sensitivity setting at some sites mean less depth? At least my staged (freshly buried) tests...yes, because in one test I lowered the sensitivity slowly until the 12x10 would no longer sound off to a 6" clad dime, and then without touching the dial I put the Ultimate on and it wouldn't hear it either, but I had to raise the sensitivity much higher to finally hear it in any way than where the 12x10 finally lost the signal further below that.
That at least tells me the Ultimate is not deeper at the same low sensitivity setting. But is it deeper at higher ones? Much like a sprint versus marathon runner, perhaps it's more "in tune" with it's designed when sensitivity is maxed out to the edge of stability? I don't know if it's deeper or not than the 12x10 in my soil, or even as deep, but I can say thus far it's seen some deep undug targets as well as the 12x10 so far, and it does indeed make deep stuff louder, although I can hear deepies just fine with the 12x10 too.
Oh, and far as separation goes...Outstanding coil for it's size in the left/right respect. Shocking really.
So as to your question of minerals, at least in my soil I'd probably give the *initial impressions* of *perhaps* penetrating minerals better to the 12x10. Not by a big margin, and if only because the Ultimate appears to need less sensitivity to stop nulling out bad ground as much as the 12x10 can seem to get away with. That ain't a huge knock to the Ultimate, because many of us know the SEFs have a stellar reputation for running smooth over the worst of grounds. And even if the lower sensitivity of the Ultimate *for your soil* turns out to be true too, that doesn't mean it's getting less depth perhaps. Got to be something to that louder target response that might have something to say on the matter maybe.
This video below I did will show you at least in air testing it can handily see through two mineralized bricks just as well as the 12x10, and also it's excellent left/right separation on nail masking staged tests, along with even it doing well on an elevated nail masking test over a dime. Now, before anybody flames me for Minelabs and air testing...I know the real world is the only place where you can really see which way the wind blows, but just the same this testing points to things to look for in the field and prove one way or the other...
Warning: I went a bit overboard with pauses and text in this video testing 4 coils, so if you don't want to read just click the bubble icon to kill all text bubbles, and if you hit "play" three times at the start it'll skip the first three longish pauses at the start to give people time to read what is going to happen here. After that the video picks up steam quick and quickly bounces between 4 coils on each masking and brick test, so the results are fresh and constant in your mind. Very interesting stuff. I know it surprised me in a few respects, which I won't spoil for anybody who bothers to watch it...
http://youtu.be/bKesj7KjcXY
PS- The Ultimate appears to have very good coin on edge ability, like the 12x10. I dug a wheat straight up on edge with the Ultimate at a spot around a big tree that I test every machine and coil around over the years. I've absolutely pounded the snot out of the 10 foot diameter around this tree over the years to try to test a coil or detector. I know I passed over that wheat 100 times before for years, as it's one of my "testing trees" to do that. Now, I haven't worked the 12x10 around that tree prior to this, but the 12x10 has already proven it's great at on edge coins. Glad to see the Ultimate also showed me it is as well, and I'm not the first to say that either...
PS #2- The 12x10 seems to "ride in and out" of instability when riding the edge of sensitivity better, like a car going down a bad road using better shocks. The Ultimate isn't out of control in that respect, especially for a coil of it's 13" width by 12 & 1/2" length size, but for sure it's a tad more of a "bouncy" ride when hitting a pothole of EMI or bad ground for a second or two. Thought that might be important to you in terms of the possibilities for your ground. I'd also add this- The 15x12 I owned was not as deep as my stock 10" coil in my soil on coin/ring sized targets, despite it running stable at higher sensitivity settings. I suspect it just sucked in too much ground stew and washed out targets at depth *in my soil*. Not sure if the Ultimate is beyond the point of no return in terms of coil size *in my soil* to see any gains in depth, but I am sure it's very close to the 12x10 at least. Who knows, maybe it is as deep or deeper as the 12x10 in my soil, but I just haven't seen that one way or the other yet to say either way.
Hope this helps...
