I like them both, and the M6 will find everything the MXT will when hunting in the Coin & Jewelry mode. I often use C&j rather than Relic mode with my MXT, even when I am hunting in more of a relic fashion and not keying in mainly on coins at old sites.
If you're mainly hunting relics then you're more likely to take to the MXT, perhaps, but don't under-estimate the M6. Both have served me well since their release, and while I like the audio Tone ID of the M6 for many coin hunting applications (and even some relic use when I can hear the low iron tone), it is the MXT that fills the niche for me as an 'all purpose" detector for most of the older sites I hunt, as well as working slowly to pluck coins and other small non-ferrous targets out of trashy sites, especially those with small iron.
An MXT with Tone ID, even similar to the M6 or "full scale" like my XLT, would be nice (and a back-light) for both coin and relic hunting. White's search coil selection is tops, to be sure, and I prefer to use a 5.3 Eclipse coil most of the time, reserving the stock 950 for more open areas, and a 12" for when I want the most overage and depth while retaining good discrimination capability.
Considering an MXT, I'd say, would be my best recommendation for you.
Monte