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.