Keith B , Jan 18, 2013; 04:39 p.m.
Don't have a Nikon 50/1.2 but have had the Zeiss for a few years. The Zeiss is excellent on "sharpness", different but equally as good--compared to similar vintage Nikon Ai(S) lenses--- on color, but bokeh might be the Zeiss' weak point IF you are super critical and particular about bokeh. Try before you buy.
One note: Based on actual samples of the Zeiss ZF 50/2 and ZF.2 50/2 that I have examined, Zeiss apparently changed the AR coating with the ZF.2 model, and not for the better. The newer coating results in a yellower color cast, and blue-colored flare where the older ZF version had more neutrally colored flare. Yes, that's right, Zeiss made it worse.