I used to have the M and A versions, and they were both excellent lenses.
There is not really a lot of difference among any of the 50 1.4s. They are
all great lenses, and I don't think you can go wrong with any of them. The
M is that it is the most common and least expensive. The K was only made
for about 3 years, so it is relatively rare.
I collected old test results from Modern Photography Magazine at
http://ca.geocities.com/spirope/pentlens.html
According to their tests, the K version had the highest overall resolution,
followed next by the A. Wide open, however, the M appears to have the best
performance.
