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The ZM 35/2 is excellent, no doubt. Some claim it is weak wide open, but that has not been my experience. The C f2.8 version is smaller and claimed to be sharper, but I personally find the max aperture to be too limiting for the 35 FL.
I've owned both. The C-Biogon 35 is sharper wide open at f2.8 than the Biogon 35/2 at f2, but one would expect that. From about f2.8 or f4 they are pretty much identical in sharpness, as I think the mtf charts show. Subjectively, I feel the C-Biogon has slightly nicer bokeh, but I'm probably hair-splitting. I use the Biogon 35/2 now, because I like the extra stop and how the slight softening of the 35/2 wide open is kind to less youthful faces. Stopped down, whew, very little distortion, great detail and color. The C-Biogon is more compact, as Edward says, so if that's important, let that guide you. At the end of the day, you can't lose between them.