If finding coins is your priority, the M6 is the better pick because of its tone-id. In trashy areas, you can simply listen for good targets, instead of getting fatigued from staring at the VDI numbers continuously. The MXT does not give you the same auditory information, so your junk to treasure ratio will be higher (worse) with the MXT.
Furthermore, threshold (which the M6 lacks but that MXT has) only provides a depth advantage in "clean" environments with few trash targets because deep targets can be identified as aberrations in the threshold hum. However, in environments that have trash (most real world environments), threshold doesn't accomplish this benefit.
The M6 is precision tuned to offer a silent hunting experience, allowing you to simply decipher the tones of treasure from the tones of trash.
Furthermore, most experienced MXT users will only coin hunt in relic mode because the coin & jewelry mode lacks the audio info of the relic mode. By contrast, with the M6, you can get top-level results by leaving the detector in the one and only mode it was designed for - coin & jewelry hunting.
If all you're looking for is modern clad, semi-old coins (40's and 50's), and want to also do some prospecting, then the MXT is the better choice. However, if you don't live in gold country, or you're primarily a coin and jewelry hunter, the Whites M6 will give you the best results for coin and jewelry targets. I define best results as finding the most targets, and also finding the deepest targets.
In addition to the M6's superior performance for coins, it's less expensive than the MXT.
I'd venture to guess the M6 "average depth of coin recovery" is deeper than any of the Whites machines (with possible exception of the multi-frequency DFX)