I have also read good things about the Zeiss 35/2.0 ... when shooting film. Supposedly the Zeiss 35/2.0 is prone to chromatic aberration when shot wide open on digital bodies like the M8 or M9 -- but since you're using a M6 body, you would be fine. Again, the Zeiss 35/2.0 seems to receive universal praise.
Funny, but I haven't had CA on my M9 or on the M8 prior. All of my CV lenses do, but not the Zeiss ones. I like the results from the Biogon, but the size is kind of large and the lack of a true focus tab bothers me (either make it with one or not... don't just provide an annoying nub). Finally was able to get an old summicron for an ok price which I prefer for its size and focusing tab. Not sure if I'll sell the biogon yet, but I'm not very happy with its ergonomics.
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