I am torn between getting a Canon 50mm f/1.4 or a Voigtlander Nokton Aspherical 50mm f/1.5 for my Bessa R.
Both seem to have overall good reputations and I could pick up either for under $300, something that may be hard for other fast lenses in LTM.
The Canon 50/1.4 is faster by 14%. The VC is a much newer lens, possibilty taking advantage of design improvements over the last 40 years. I probably can get the VC in slightly better cosmetic condition or without any traces of oil on the aperature.
You have to be careful with older Canon lenses that the rear surface behind the aperture is not damaged. If you go that route, be sure to check is and have return priviliges.
The F1.5 multicoated vs the F1.4 single-coated is not going to be much of a real difference.
I have the Canon 50/1.4, and like it very much. It is a big lens, as is the Nokton.
Sometimes we have a tendency to get to wrapped up in speed differences.
14% is less than the tolerances used in calibrating shutter speeds. Typical is 20% and at higher speeds, 500 & up 50% is considered within tolerance.
That's what I was going to say. The real world difference between 1.5 and 1.4 is negligible and is more of a marketing thing than anything that will show up in photos ... or be visually apparent in photos.