Between those 2, I'd go for the Canon.. The Zeiss 1.4 isn't as good as some of their other lenses in terms of sharpness. The Zeiss 2.0 Makro-Planar is crazy sharp, but 1.5 stops slower than the Canon.
The 50mm f/1.2 is a temperamental and tricky lens. It's common for them to backfocus within 4 feet. The Zeiss, although not quite as crisp wide open, has just about perfect mechanics. Most people who buy the Zeiss, buy it because they enjoy the mechanics of it. They're just really fun to shoot with! Hopefully in about a month or so, i'll buy one to stick onto a Nikon F3