The difference is the benefit of the additional Ground Grab touchpad on the MXT Pro. This eliminates the need to pause, reach-over and toggle back and forth with the control-mounted Track/Lock toggle.
With the original MXT and the M6, you can adjust a Ground Balance and 'Lock' the setting.
The detectors are still 'tracking' or 'reading' the ground mineral changes, just not updating the GB setting because the 'Lock' position has it held.
If using the M6 or MXT, or even the MXT Pro in the Locked GB setting, and you determine the GB setting might be 'off' due to a change in the ground mineral make-up from where you initially balanced and locked it, you can sweep the coil over a clean (metal-free) spot of ground, pause and hold the coil motionless on the ground, reach over and move the toggle from 'Lock' to the Ground Track position, then back to 'Lock' and it will update the set GB with what the tracking had determined.
If you are right-handed this can be a pain to reach over and tinker with the toggle, but it's doable.
The MXT Pro, however, features the Ground Grab touchpad which serves two purposes, as Nauti Neil mentioned.
1.. If you ARE in the Ground Tracking toggle setting and want t hold momentarily hold or lock the GB and halt the Auto-Trac update, just press and hold the GG touch-pad.
2.. If you are in he 'Lock' setting, you can simply sweep over the ground, pause, and press the GG touch=pad and it's updated, or you can press the GG touch-pad and bob the search coil from 6" to the ground a few times and get a proper GB.
The M6 does the same thing the standard MXT does, you just have to manipulate the GB toggle. I haven't used the M6 as much since I switched from the M6 & MXT to just the MXT Pro, but I will double check and verify this today just to be sure I haven't erred.
Remember, too, that the M6 was intended to be a simplified model with MXT performance, but intended for many Coin Hunters who like 'simple.' It might be that it was easy to simple eliminate having the tracking and updating information in the simplified M6 manual.
Monte