Why would there be any optical differences whatsoever? Given that the camera body is merely a light-tight box, and everything that hits the film passes through the lens, there will be absolutely (with one possible exception, just a moment) zero difference in image quality between Leica M film bodies.
The one possible exception (in very limited circumstances): of the three bodies you mentioned, the only one that comes with a 0.85 viewfinder as an option is the M6, which model I happen to have. Because of the greater magnification and the longer effective base length, on rare occasions with a 35 or 50mm lens I *might* nail focus better with that body than with an M2 or M4--but those occasions would likely be infrequent indeed. And as others have mentioned, if I use a 90mm or a 135mm (both of which I do use...rarely), I probably will in general be more successful at focusing sharply with an M6 0.85. If you have a standard M6, it has the same 0.72 magnification as the M2 and M4, so you'll see no difference. And you could reverse that and say that even my 0.85 suffers the same defect as all factory M6s, in that it's missing a rangefinder condenser, which can cause white-out of the focus patch that definitely interferes with focusing.
Don't stress it: figure out which camera is going to be most ergonomic and pleasurable for you to use--there are clear differences between the M2, M4, and M6 on that front--and use it, and don't worry about any phantasmagoric "differences in image quality".
The one possible exception (in very limited circumstances): of the three bodies you mentioned, the only one that comes with a 0.85 viewfinder as an option is the M6, which model I happen to have. Because of the greater magnification and the longer effective base length, on rare occasions with a 35 or 50mm lens I *might* nail focus better with that body than with an M2 or M4--but those occasions would likely be infrequent indeed. And as others have mentioned, if I use a 90mm or a 135mm (both of which I do use...rarely), I probably will in general be more successful at focusing sharply with an M6 0.85. If you have a standard M6, it has the same 0.72 magnification as the M2 and M4, so you'll see no difference. And you could reverse that and say that even my 0.85 suffers the same defect as all factory M6s, in that it's missing a rangefinder condenser, which can cause white-out of the focus patch that definitely interferes with focusing.
Don't stress it: figure out which camera is going to be most ergonomic and pleasurable for you to use--there are clear differences between the M2, M4, and M6 on that front--and use it, and don't worry about any phantasmagoric "differences in image quality".