It is always interesting for me to read threads like this one and see that, often enough, optical quality of a lens is not the primary reason for choosing that lens. Instead, the perception of built quality and “feel” are often the determining factors. Frequently, weight of a lens is associated with its built quality. There is only one thing in the whole wide world that fulfills the criterion “built like a tank”. Looking at the MTF diagrams, distortion and vignetting curves for the two lenses, biogon appears to be optically a better lens wide open and closed down. It is hard to tell from looking at these diagrams how a lens will render an image on film or sensor. My impression is that Zeiss lenses render color more accurately than Leica lenses. Because of its optical qualities, I chose a biogon. It also fulfilled, by a big margin, my other most important criterion for choosing a lens: “the price”. My pockets are only so deep. Both lenses are built well enough to do the job for me.