It seems Zeiss's new 55mm f/1.4 lens which promises to be as sharp as current Canon 50mm lenses are at f/5.6 wide open is making a lot of news (50mm lenses tend to fall far behind other primes for image quality, delivering what 24mm and 85mm primes deliver wide open at f/4.0). In my own quest for a great 50mm prime I've looked at every 50mm prime thats ef compatible or ef adaptable made since 1970 and all deliever this mediocre image quality (though being 55mm is likely key in allowing the iq).
So Zeiss has cracked the high quality normal prime mystery that has eluded all other manufactures for decades.
Well, I'm not sure it is as astounding as it may sound. A diffraction-limited lens at f/5.6 pumps out 123lp/mm of spatial resolution at the sensor. Theoretically, a diffraction-limited f/1.4 lens could pump out 494lp/mm of spatial resolution at the sensor! That is a monster difference, and indicates that even Zeiss' 55mm f/1.4 lens is extremely aberration-limited wide open.
There is also the difference in goals. Canon's 50mm primes, particularly the EF 50mm f/1.2 L, are designed to produce softer focus. The 50/1.2 is explicitly designed to retain a certain amount of spherical aberration as it produces very nice boke blur circles and a soft glow around OOF elements near the plane of focus when the lens is used wide open. People who are looking for a lens like that buy the Canon EF 50mm f/1.2 explicitly for that purpose...pixel-level sharpness isn't a concern in such a context.
If you want extreme sharpness, you would probably be better served stopping any one of these lenses, Canon or Zeiss, down to f/2.8, where spatial resolution should be approaching it's highest, and DOF will still be fairly thin (and more manageable, allowing you to get your whole subject in focus and nicely sharp...which is kind of the antithesis of why you would use an ultra-fast lens in the first place). If you stop down like that, you might as well look into the new EF 24-70 f/2.8 L II, and use it at 50/2.8. It sports an MTF that neither the EF 50/1.2 nor the Distagon T 55/1.4 can touch, and would pound out far better sharpness wide open than you would know what to do with.