The MXT I have easily detects a quarter at 12" or more in an air test. And I guess a skillful operator might be able to equal the air test in mild ground.
In the more mineralized ground, the ID might be wrong or missing at 6 to 9 inches & it might not detect deeper than that. My ground has fairly high minerals & I only dig what visually IDs as coins, so the deepest I've got coins is 7". But 7" is the deepest I've ever got with any detector.
Besides the ground, there is differences in depths with coil size & type. I haven't tried the larger than stock coils.
The MXT has newer(better) ground tracking than the DFX, plus ground lock & salt mode. Also, the MXT beats the DFX on small gold. About 99% of the MXT users sem satisfied. HH, George (MN)