A very simple way of comparing the 50 Cron with the ZM 50/2 (certain aspects anyway) is to look at the MTF charts:
Leica red, Zeiss black. In the f/4 chart the Leica is actually at f/5.6.
It pretty much shows the usual Leica & Zeiss difference. Zeiss usually has less field curvature and less astigmatism. Leica usually on the other hand prioritizes minimizing SA & CA.
There are exceptions, but as a rule of thumb modern Leica lenses are superior wide open and in the center part of the frame while Zeiss ZM tend to be a notch better and more consistent across the frame stopped down. Zeiss also puts more effort in making the sagittal and tangential lines as parallel as possible all the way to the corners. This means that the blur spread is even in all direction giving a stronger impression of crispness.
Zeiss rendering style is more uniform across the entire product line while Leica lenses show more diversity.
A good case can (unfortunately) be made for having an overlapping set of Zeiss and Leica lenses as they're good at different things.