Go with the MXT if you can afford it, the extra features are worth it in the long run. This is just my two cents after owning both detectors.
As much as I liked my M6 that I had I did upgrade back to the MXT as I had one before and made the mistake of selling it. The M6 is a non threshold silent search detector and a fantastic coin & jewelry detector, but it does not have the prospecting mode and saturation (SAT) adjustments needed for serious gold nugget hunting. The MXT is a way better unit for this application. To be honest with you I like the MXT a lot better over all. I like a threshold based detector myself like the MXT over a silent search type detector like the M6, this is just my personal preference. Also the MXT has the relic mode with the all metal multi tone or mixed mode option, its not a 7 tone ID like the M6 but it is very useful in a lot of situations. I love it in the relic mode when hunting in iron trash and regular heavy trash areas for coins. I also like the alternate relic mode trigger forward position, the relic mode has a lot of uses.
Back to the question you ask, the MXT can tell the ground make up and react when hunting for nuggets it will give you saturation (SAT) speed options and a reading in numbers from a low to high for ground mineralization on the screen. The prospecting mode on the MXT is modeled after the GMT, Whites top end nugget detector, the M6 does not have this nugget hunting prospecting mode with the features needed for nugget hunting at all.
For nugget hunting, Relics, coins & jewelry hunting go with the MXT. For coins & jewelry the M6 is great if you like a silent search unit with tone ID, also the M6 will do for some relic hunting with the disk turned down if needed. I think for the extra $100 bucks the MXT is a better choice with all it has to offer and would be the way to go if you plan on any nugget hunting at all.
If you decide to get the MXT Get the book the MXT Edge as this will help big time understanding all the MXT has to offer in the hands of an experienced operator with all three operating modes, coin & jewelry, Relic and the Prospecting.
IN SUMMERY
As much as I loved the M6, I feel the MXT is a better detector hands down due to the extra features and the extra performance allowed by them in different situations such as taking advantage of the relic mode when hunting for coins in certain situations, the relic mode shines in iron trash and heavy trash areas. The prospecting mode for nugget and small nugget hunting because of the (SAT) or adjustable saturation speed and the superior ground tracking system works great for nugget hunting and of course the good old coin & jewelry mode, truly three detectors in one.
Best of luck you will like which ever detector you choose, both the MXT & M6 are great units.
Bill G