I really would like a 35mm for my M4 and Leica glass is pretty much out of the question based on what their various 35mm lenses seem to command. The Voigtlander 35mm 1.4 and 2.5 are both pretty reasonable and new besides. While I won't say I'll never shoot color my interest in film cameras is, so far, strictly b&w.
What are the thoughts on these two lenses or the Zeiss Biogon?
The CV 1.2, 1.4, 1.7 and 2.5 lenses all have good reputations.
The ZM Biogon
s (2.0 and 2.8) are a little bigger and a lot more expensive than the equivalent CV lenses, and in general they show a higher level of correction for certain aberrations.
In particular, they have very flat fields and almost no geometric distortion, and they
might be better (higher contrast and resolution) at wide apertures.
For most photos, these differences will not matter. I use the ZM Biogon-C 2.8 because I usually shoot at 2.8-5.6, it's small, it has almost no distortion, and it has almost no defects. I just don't have to think about it. I also am in love with the bokeh of the Biogon-C at 2.8. But that's a very subjective preference.
I'm certain that my photography would not suffer if I had to switch to any of the CV 35's. Among the CV's my first choice would probably be a used Ultron 1.7, second choice would be the 2.5.
I should add that there are ergonomic differences (focus tabs or not, etc.) between the different lenses that are not a big deal to me, but for some here are make-or-break.