Well as they say there isn't one perfect detector made. So my first question is what type of sites will you generally be hunting the most ? I use the M6 and MXT Pro because of their ability to handle iron dense sites such as ghost towns, old homestead sites etc. They will do great on Monte's Nail Board Test. Since those are the sites that I hunt the most, I am not really concerned about the depth. I use a smaller then stock coil ( the Eclipse 5.3 and the Detech 5" round DD) and each machine has the ability to lock the GB.
In more open less trashy areas I feel my XLT set up with the proper program will do better when it comes to "Depth" Using a Royal 800 coil I was with Monte in a Eastern Oregon town. I installed his "Bushwhack" program and we recovered two war nickels and a 1939 nickel. Monte measured each one and they ranged from 7 1/2 to 8 1/2" deep. Although at that depth the VDI numbers were not correct, they did however give a nice repeatable beep.
I feel that one machine is not better then the other, rather they compliment each other. Where one may lack a bit in one area the other will compensate.
As far as coils for the M6/MXT's I have the 9" Spider Coil a 5.3 and the 5" DD. Of all the coils that I have tried and put through the paces, I got the worst performance from the 10" D2 coil. It was just too erratic and VDI numbers constantly jumping. If I felt I needed a "big" coil I would use my 9" spider or sometimes use the 10 x 12 SEF . The 12" round Concentric would also do good, but is a bit of a big clumsy coil. Had they placed were the rod attaches to the coil more towards the center of the coil, like on the 10 x 12 SEF, it would have balanced much better.