Like Peter, I've heard that the M6 and not the M6TTL can be a potential source of problems because of the age of the electronic components. The M7 is still too young to fail (although leave it to Leica traditionalists to complain about the way the wind blows and what's Leica and what's not), but I've heard people ranting about film cassettes not falling in their hands when they take off the bottom plate of the camera.
In any event, I ended up buying a TTL by sheer dumb luck the first time, and on purpose the second time, after reading about how older M6 bodies start needing further electronic servicing.