the zeiss has a hard infinity focus stop so i would have guessed that manual focusing for astro work couldn't get much easier. It's got complex distortion, and color shifts at the corners of the frame, and according to photozone.de at f2.8 and smaller the canon is at least as good or better. But of course the canon can go 4x as fast if needed. actually, go to 16x9 .net? and see the old canon 24mm 1.4 match the zeiss, and that's from guys who think mounting a nikon 14-24 on your canon is reasonable!
I had a 17-40 and 16-35. now i'm rocking a samyang 14 and it's pretty cool. wish it had a hard infinity focus, and electronic aperture control, but i'm managing. old school focus distance guessing at f2.8 on moving targets in the dark is actually working pretty well. i have no doubt there are dogs out there, but mine 14 is better than my 16-35 and 17-40 were. (IQ wise that is)
That all said, it sounds like you have some money to spend and don't mind having a few lenses at what i would consider very close focal lengths, so maybe you should pick up a zeiss. either way i doubt you'll lose much money on the deal, and that way you'll know for sure how you feel about it.