Although they both provide wide open performance, the two lenses essentially differ in the type or style of image that will result.
The Voigtlander 50mm f/1.5 and 40mm f/1.4 Nokton lenses will provide a much greater level of clarity or sharpness than the Canon 50mm f/1.2. The Canon is able to open aperture blades slightly more than the Noktons, but this is of course only applicable to low-light exposures.
Indeed the difference in aperture capabilities is so slight that you may never notice the difference unless using the camera in AE mode and then reading the EXIF data to determine the precise shutter speed that was calculated by be onboard light meter, i.e. in manual mode you must shift between preset exposure timings, whereas the onboard processor in the camera's light meter has the abilty to implement a finer gradation of exposure timings.
The aim of my test shots was to show the performance of the lens in a low-light environment. In a situation where shadowed light is most prevalent, the Canon is able to provide a sufficient level of detail to ensure that the resultant image displays the more important features of the subject that will define the individual character of each object.
It is also pertinent to note, that the natural contrast of a darkened environment essentially negates the softness of image that would be seen when using the Canon in strongly lit environment or in normal daylight.
So the decision as to which lens you need, is more a question of the kind of image that you wish to develop. If you know that you would be unsatisfied with the softer image that results from using the Canon, then you should go for one of the Noktons. If you are more concerned with the content and creativity of the subject in your images and will only use this lens in low-light situations, then you can save yourself the extra cost of buying a newer lens.
As regards the price, if the lens really is in excellent condition, then $300 is not such a bad price to pay, especially if you consider how long you might wish to keep the lens. If the lens is only in a fairly good condition, then I would strongly suggest you wait upon another opportunity. Alternately, if you can wait for another two or three months until you can save the extra money for one of the Noktons, you would likely not be disappointed, and there is always the possibility that during this time any of these lenses may become available on the used market. The key point here, is to only accept the best that you can afford, and the magic ingredient for success in this and indeed in all matters, is patience :angel:
Peter