I bought both the Nox 800 and the MMK. Different tools for different jobs. I really like the MMK. The pros over the Nox to me are that the MMK Is more ruggedly built, is easier to navigate thru the menu, better balanced, and I prefer the sound or tones better. The MMK has more coils available and the coils are much less expensive. Both detectors work very well as "overall" detectors. The reason I have both is that there are a few areas that I prefer one over the other. This is my take on it:
The MMK - Outstanding at pulling non-ferrous targets out of iron infested sites. Super fast with great audio info. This is my go-to detector for Old lots, ghost towns, railroad sidings etc. I think the MMK with a small coil, is also outstanding at pulling targets from modern (aluminum) trashy parks.
The Nox on the other hand really loves round targets and the MF modes seem to get a little better depth so it's my Deep silver hunter and works well at the beach being able to go from the dry sand to the wet without issues.
I have a number of coils for the MMK, both DD and concentric, and I have the 6" for the Nox. Can the Nox with the 6" coil pull targets out of old lots? Sure it can. Can the MMK be used at the beach? Absolutely. Can a nail be hammered with a crescent wrench?
I had the Impact also. Really liked it. Along comes the MMK that would do everything I asked of the Impact, plus be lighter weight, waterproof, and less expensive. Bye, bye Impact.
If I could only have one detector I'd be happy with either one. Fortunately, I can have both (plus a couple more). If really pressed, I'd go with the MMK because I hunt more in trashy areas, don't get to the beach that often and where I live in AZ, there's not that many old parks to pull silver.
JMHO