Originally Posted by jja
One important consideration is your metering needs (coverage and accuracy). I believe the F3 has a semi-spot meter patch, compared to the larger patch of the M6 ...
The Nikon F3 meter pattern uses the same 13mm circle as the FM and FE series models. Nikon uses a centerweighted pattern and the weighting is 85/15 from center to edge in the F3 compared to the F/F2/FM/FE 60/40 weighting. (Yeah, I know the spec says it is 80/20, but measurements with a point light source on my F3 established quite conclusively that it was actually 85/15 weighting. I did the testing because I was curious that my FM and FE cameras gave different readings when pointed to the same scene with the same lens...)
The M6 meter pattern is a limited area pattern, not a centerweighted pattern... It reads the central white spot on the shutter curtain that's about 12mm diameter, so has a more well-defined cutoff than the Nikon F3.
In truth, and for all practical purposes, they respond so similarly there's hardly any difference between them in use.