Very well stated Steve, I see things pretty much the same. BTW, I do own and use an MXT for mineral searching, but have not used it for gold hunting yet.
The nice thing about MXT, as you point out, is that there are choices as to how one elects to use discrimination. In the relic mode, trigger in the center position, you decide just how much iron discrimination to dial-in for the tone change, and deal with each individual signal with further investigation or not, depending on your frame of mind, weather, or bugs at any given moment. But at least you are aware of the signal and make a conscious choice. Or check out each signal using the prospecting mode.
Where repetitious iron debris or hot rock signals that by sheer numbers are a source of irritation, one might elect to run in C&J or Relic (trigger forward with disc dialed-in) with a sufficient iron discrimination level to break-up/eliminate such signals to save your sanity. You accept that some small gold will be lost, but at least its an alternative to giving-up and walking away. This is a benefit of having a full discrimination circuit as opposed to only tones/visual ID. Moore Creek basaltic hot rocks might be a good example where an audio disc circuit would be preferable to say, the prospecting mode's iron audio ID feature grunting away and driving you crazy.
Its handy to have that versatility utilizing iron discrimination. By comparison to MXT, the F-75 does not allow one to define the tone change by arbitrarily setting the discrimination level. Its iron tone represents the full iron scale, similar to MXT in relic mode with trigger forward and disc set to zero. However, the F-75 does allow one to utilize the iron tone in conjunction with an arbitrary iron discrimination setting dialed-in. For example, one can elect to set discrimination at a setting of "3" in JE mode as a means of quieting spurious ground noise, but also have iron tone (2F) activated. All iron signals ranging from "4" up to and including "15" will be heard as an iron tone, but you also benefit by reducing (discriminating) ground noise that can result where the F-75 in zero discrimination may see the ground to some degree as a "target". This unit can use all the help it can get in dealing with various noise sources, and this is a good example of where using this iron disc/tone configuration also aids with overall stable detector performance. Of course as you say, the use of iron tones is really only practical in situations where material can be removed easily. Otherwise, for instance with coinhunting, one faces potential masking of desirable non-ferrous targets, either because of depth in conjunction with magnetic ground minerals, co-located ferrous/non-ferrous targets, or due to the effect of disturbed ground.
MXT and F-75 are similarly both deepseeking capable (aside from their all-metal modes) utilizing small iron discrimination mode settings over disturbed ground (loose "stirred-up" mine tailings for example) compared to some detectors that perform poorly over such ground using any amount of iron discrimination, for whatever reasons. JE is the only F-75 discrimination mode that will allow utilizing effective small iron discrimination while retaining deepseeking capability over disturbed ground and yet still sensitive to small nuggets of a few grains. The other disc modes must be used in "zero" discrimination to achieve similar depth performance over this type of ground.
The stat all-metal mode is really something if you can make good use of it. Over my ground, unfortunately, the sensitivity has to be reduced down to at least the "60" range or less in order maintain a half-ways practically useful ground balance and reasonably stable threshold as the coil is swept over even slightly changing ground. I find I have to be circumspect as to how far above the ground I manually retune the unit even at the "60" sens setting to ensure the unit remains practically operative. The stat mode is so sensitive to ground minerals, but is also extraordinary in depth capability. But as I noted, due to the much-reduced sens setting, I