I'm thinking the 6D would make a great "reportage" camera. It isn't a great big camera. You could put it in the extra quiet shutter mode and shoot with the 40mm pancake lens. The 6D is a heck of a lot quieter than my old M8 and unlike the M8 the 6D is full frame and works great in low light.
What do you think?
I opted for the 5dIII instead because I have 2 small fast kids and wanted max. AF. I am quite happy with the camera. The quiet shutter is nice.
However I never found the shutter sound of M8/M9 to be a problem.
And a 6d is still considerable bigger than a M8.
If you compare the M8 with the 6d I would say:
6d advantage:
+much better IQ when shooting ISO 1000 and higher
+AF
+ more MP - advantage for big prints
M8 advantages:
+ IMO up to ISO 640 more "pop" IQ
+ smaller (specially the lenses) (the Canon 40 is maybe very small, but its an exception and I doubt you can compare its optical quality to the Leica/Zeiss/CV lenses)
I still use both systems - the Canon I use when I want long reach or need fast AF or in low low light. However I would prefer a fast 50mm or 35mm over the 40mm pancake.