Particularly, I am interested in the performance on film for Canon leica thread mount (LTM) 50mm lenses versus their FL/FD SLR lenses, in aperture size 1.2/1.4/1.8. Their performance on full frame digital bodies like the A7 may also be valid with respect to things like sharpness, contrast and bokeh.
The main motivation for my question is the difference in prices these lenses commandeer on the used market, you can get a 50mm 1.8 SLR lens for less than $50 but the LTM one is usually three times as much.
Given that most of these lenses are probably six element double gauss designs, is there any intrinsic property that makes the LTM worth more/better than SLR version? For example, from this link, I quote this about the 1.4 LTM:
Impressive image quality for a basic design (I have found this lens better at f/1.4 than my copies of FD SCC, Takumar 7 and 8 elements, Minolta MD 50's)
What can be said about the comparison between the Leica Summicron f2 vs Summicron-R? Or the Zeiss planar 50mm f2 versions made today for DSLRs vs Leica M mount.