The DFX runs @ 3khz and 15khz. You can run them both at the same time in Best Data mode or Correlate Mode. You can also select either of them individually to hunt with. You are right, the conditions; mineralization reading, mineralization strength, EMI, desired target, trash factors all enter into the decision on which frequency setup to run with. 3khz is more sensitive to high VDI targets (pennies, dimes, quarters, halves, etc) and 15khz is more sensitive to lower VDI targets (nickles, jewelry, etc). Using just one frequency offers the ability to notch out trash with more percision than using both at the same time. Running them both at the same time has many advantages but can loose a little bit of depth. Whites selected these two frequencies because they are the most sensitive frequencies for the range of targets above. If you had more frequencies available would that mean that your detectore would be more senstive in areas outside of the targets ranges above? I'm not sure I'd want it to be more sensitive in other areas, trash for the most part.
I've only been MD'ing for 6 months and the DFX is the only detector I have owned so I can't comment on the other detector.