I've got the nikon 50mm f1.2 that most compare with the voigtlander. Personally I can't see me swapping over as the nikon is great. Both have the usual fast double gauss attributes. They both suffer from coma flare and slightly busy bokeh wide open and both sharpen up pretty fast when stopped down.
The voigtlander is slightly sharper in the centre wide open due to higher contrast, but it's a slight difference not a big one and the rendering will be subtly different due to the longer focal length. Personally, I'd get whichever was cheapest. I would also point out that the difference between a current AF 50mm lens isn't likely to be all that great either. I went for the 50 f1.2 over the voiglander and an AF counterpart as it can do f1.2 which they can't and it is well known for being exceptionally sharp at f2. (more than any MF nikon 50 and more than the older AF-D's, on par with AF-S)