The Canon 7 have meters which may mean more maintenance ($) the cadding may be thinner, P and earlier are built like Ru tanks, the 7s like UK tanks.
The pre 7 Canons do have different viewfinder (the frames are not projected like M's) optics - you have to look through them in dim light before you buy - finder problems also occur e.g. M3, and M4-2, cleaning a M4-2 is cheap, the M3 and early Canons may be more of a (>$) problem.
Both my VI-L and P have tarnished optics but last Sat two M owners spent ages fondling them in dim lighting and said you cannot see the rangefinder frame like a M but the focusing coincidence is just as easy, their reference M2 had just been CLAed. The rangefinder is subjective you need to do the dim light test yourself to see if you are happy, with both the body you are buying, and the Canons P double image, it is not like an M, and can be off putting initially.
But to answer your question I am not troubled by flare & later Ms than mid M4-2 can also flare badly, but there is an upgrade available. Take the body you are buying out into the sun, even if it is an early M4-2.
The Canon lenses tend to be superb even after 40 years, but they also may need an internal clean, if it needs a clean ask for a discount. the Ru lenses are ok alas the j12 I have does not fit either body I have, it has also a tendancy to flare, but it is real cheap.
If you have a local brick camera shop want until they have a Canon in the window, they dont sell that quickly, if you like it buy, buying one on e-bay more risky, if it shows signs of damage and the shutter will have more wrinkles then me, ask for discount, the shutters are normally ok wrinked. Dont stick a finger in the shutter after you buy...
The Canon kit tends to be a lot cheaper than leitz, fewer collectors.