I've heard complaints that the shutter-curtain lever on the 7ii is easily broken plastic, vs. the more-robust metal of the 7.
Never used or owned the 6. I love my 7. I have 50-80-150 lens set, find myself wishing for the 65 instead of the 50. 90% of my shooting is with the 80, almost none with the 150, and a bit with the 50. All are stunningly sharp. The 50 vignettes a bit wide open.
Have owned the 7 ca 1-1/2 yrs; have had the rangefinder adjusted twice, once as part of a CLA it was due anyway. Also had them add a strap lug to the right upper deck, as the 7ii has, so that the camera would ride horizontally on the strap like the 7ii does., rather than hanging sideways by the lugs that attach at top and sole plates along its left side. And they did the 7-->7ii finder upgrade; well worth the $70 for the improved brightness. It essentially adds a polarizer layer to the viewfinder window, quite an improvement.
Meter accuracy is good, once I stopped trying to second-guess it and just let it do its thing, with sparing and intelligent manual overrides. (90+% of my shooting is C-41 color.) I can get decent hand-held sharpness down to 1/30; beneath that, I just can't make it happen free-hand without support against something.
The Mamiya 7 is the one camera I'd rescue from my burning house, if I could only save one.