First thing to appreciate is that the M6 is based on the MXT 'platform' which means they share the same excellent Auto-Trac circuitry, Discrimination, Gain/Sensitivity and therefore depth and general performance.
They are in a different physical configuration, with the MXT using the rod/grip system like the XLT and DFX, and the M6 is in the more compact 'S' rod design that was popular with the Classic SL series. I like them both.
So, what differences might these be between the two of them?
Search Mode Choices:
M6: Designed especially for the coin hunter, the M6 features a Coin & Jewelry 'mode' similar to the MXT. That is, it is a single-tone audio response. Accepted targets respond with a beep, and rejected targets are not heard. Pretty darn simple. However, the M6 does have an audio option when you flip the trigger-toggle forward that activates a 7-Tone audio ID. You can flip the trigger forward and hunt with Tone ID all the time, or you can just select it when you want a little added audio information about a located target. Personally, I use the Tone ID function sometimes, but definitely not all the time as it can get a bit "busy" in some environments.
MXT: The MXT is the true multi-purpose detector featuring both motion Discriminate operation and All Metal operation. It has three (3) search mode option:
Prospecting, which is an All Metal mode and is supported by a variable S.A.T. (auto-tune) control.
Coin & Jewelry, which is a single-tone audio mode similar to the M6. The difference is that it lacks a Tone ID option. It does provide a level or "notch" rejection for some of the older-style pull-tabs which is factory-set (you can't control it), and is attained by flipping the trigger-toggle forward in this mode.
Personally, I have very little use for this feature because most of the modern pry-tabs on beverage cans tend to 'read' similar to our US 5