I do not see where there would be a difference in the salt, though I have not taken either to a salt beach. Iron, I like the 2 tone and adjustments of the MXT so I am partial to the MXT for relic hunting. However that is not saying the M6 is not capable, it's just a preference that I am used to. They're frequency is the same. If I were mostly coin and Jewlery hunting with an occasional relic hunt, I would go with the M6. Since I relic hunt often and occasionally coin and jewlery hunt, I like the MXT. Hope this is not to confusing and is my preference.