Well we're talking Leica M's here, so the truth is that you'll probably love either camera, and if you don't, you'l probably get all your $$$ back whan you resell it. One point to remember is that a M6 Classic can be anywhere up to 20 years old, where as the TTL can't be older than 6. That's a baby in Leica years.
If you haven't guessed already, I prefer the TTL even though I'll never use any of those puny Leitz flashes (Vivitar 283 r00lz d00d). the meter's more sensitive by one stop, and I like the meter display better. The TTL has a dot between the two arrows, a la... "> 0 <", which allows it to indicate a half stop over or under exposure (when the dot and one arrow are both lit). I find this feature handy for applying quick exposure compensations. The Classic only has the two arrows, both light up when the exposure is correct.
Another thing. If the meter doesn't have enough light to operate, both arrows and the dot will flash (not sure if the Plane-Jane M6 does this too). I've discovered that this is a real handy lens-cap-is-still-on indicator.
Now, I am very sorry to have to disagree with our esteemed colleague from France, but the arrows in the M6 TTL viewfinder really are intended to indicate which direction to turn the aperture ring and shutter dial - that's why Leica changed the direction the shutter dial turns on this camera (as well as the M7 & M8). I will agree that in the M6 Classic, they are not intended to perform this function at all, and are merely centering symbols.
Seriously though, I've handed my camera to a number of people who've never touched one before, told them to just turn the aperture or shutter dial in the direction of the arrows until the dot lights up - they've all got it right instantly. I find it's really fast and intuitive to use - if you don't spend too much time thinking about it 😉.
The larger Shutter dial is a slight improvement in ergonomics, but I've never had a problem with the smaller one -a solution to a problem that doesn't exist IMHO. As for the quality of the Viewfinder, I think it's pretty much the same between the two. My TTL RF flares out occasionally, so if it has any improved coatings, they don't make much difference.
The downside is that the TTL's more expensive, the shutter dial turns in a different direction from all previous M's ( but the same direction as almost all subsequent ones) and it's really not quite as nice looking, what with being 2mm taller and having that honkin' big shutter-speed dial up top.
Having said all this B.S., Much as I think the TTL is a (slightly) better camera, a skankin-good deal on a M6 (especially one that's has the RF flare problem fixed) would trump a TTL at regular prices any day.
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David Dutchison