They are very similar. The Whites M6 has the same circuity and operates on the same frequency as the MXT but the m6 is programmed to permanently operate in a coin-hunting mode and doesn't have the gold-prospecting or relic hunting modes of the MXT. However, the M6 borrows from the MXT's relic mode and even improves upon it by offering an excellent tone-id that really allows you to pull the deepest, faintest sounding, signals out from all the chatter, even with zero discrimination.
The M6 can also use the same search coils as the MXT, and in my experience will get slightly more depth on coins (on average) than the MXT will. The M6 is optimized to permanently operate without a threshold (the continuous hum you hear with the MXT) but this doesn't seem to sacrifice sensitivity as the M6 appears to operate just slightly below the "point of humming" so even the mostly slightly sensed metals will make a sound.
Overall, if your focus is coin-hunting with the occasional relic hunting, you will love the Whites M6.
I got mine recently and am pulling out old coins from a park I had worked heavily with the MXT. My loot from my last three outings is 22 wheat pennies, 5 silver rosies, 2 mercuries, and more clad than I can count. I can't wait to take this thing to some virgin hunting grounds!