Hi MEV,
Your question is a very simple one. However, it is hard to give a simple answer when we do not know your preferences in detector behavior nor how you plan to use the detector.
If you plan to do any nugget prospecting, then the answer would be the MXT Pro is the better detector. The MXT Pro has a Prospecting mode that works very well and the M6 does not.
If you plan to do Relic hunting then the MXT Pro would again be the better choice. An M6 will find relics but since the MXT Pro has a separate Relic hunting mode, the MXT Pro has the advantage.
For Coin & Jewelry hunting deep targets, the MXT Pro still has the edge since it has a Threshold. The MXT Pro threshold Whisper will alert to deep targets that the M6 will miss. And threshold nulling on the MXT Pro will alert the user to slow down and / or switch to a smaller coil as the trash level increases.
If what you are looking for is a quiet machine primarily for Coin & Jewelry hunting with an occasional Relic hunt, then the M6 might be best for you. It will produce LOTS of great finds for you. However, you need to be aware that the M6 will not find targets quite as deep as the MXT Pro. And because the M6 has no Threshold to alert you about the trash level, you may swing too quickly through a high trash area with an M6 and miss good targets.
By the way, you can adjust the threshold to silent on an MXT Pro and it will operate exactly like an M6 silent search. You cannot adjust an M6 to operate like an MXT Pro.
Both are high quality, fun to use, detectors. And both will produce lots of good finds. The M6 will deliver more peaceful hunts.
Good luck and have fun.
Bob