Comparison amongst my M-bodies, all in excellent condition, some in use since at least four decades; just my humble view of course:
Viewfinder characteristics including flare:
M3

.............. M6 0.85 Classical
M2, M4

............... M6 .72 TTL
Smoothness; feel of quality:
M2, M3, M4

.............. M6
Going to Africa for 6-months, which one to take with a couple of lenses?
M4
Have to sell, which one first?
M3, M4

(all others

)