I would qualify Optikus's reply : manual focussing on an slr is much more difficult with wide angle lenses than with longer focal lenses. That's because things don't "snap in focus" in the same way, due to the wider dof. You may see things in focus on your viewscreen, which provides a small enlargment of your picture, but when printed at a significant size, you find out that your focus plane is not where you expected it to be.
Another important thing is that you need to install the viewscreen specifically designed for manual focussing on your camera. The original viewscreen sacrifices accuracy to brightness. I had this viewscreen on my 5D1.
I have the 24/1.4 mkI. I acheive better focus with the autofocus than manually with this lens. Conversely, I can equal the autofocus manually with the 135/2.0, provided I have enough time to focus carefully.
I used to have the zeiss 21/2.8, Y/C system. A great lens, without any doubt. However, remember that sharpness is a combination of good optics and accurate focussing. Personally, I would rather take the canon 24/1.4 II, because I would not trust myself for focussing accurately with a manual focus wide angle lens, especially for environmental portrait, at wide aperture. Of course, at smaller apertures, the wide dof largely solves the problem.
My reasoning would be different if you were to choose between two short teles.
If you can, try out the zeiss, and see how easily you can focus on a face, at full aperture, in average light conditions. If you hit perfect focus, that your eyes are much better than mine, and this lens is for you. Otherwise, you proabably better off with the canon.
Hope that helps.
