Meanwhile, seems I was wrong on the old versions of Zeiss 20mm, there were only ever f/2.8 and f/4.0 versions, no f/2.0 (maybe I dreamed that). And they came in m42 as well as C/Y mounts.
Anyway, read this thread for opinions on the f/2.8 vs f/4.0, third post down is useful. In short, the f/4.0 is apparently sharper, but not great, somewhere I've read that the Nikon 20mm is better than both.
Also, I read something yesterday but now i've lost the link, comparing the 20mm Zeiss to the 16-35L (mk1 i think). The 16-35 won easily for wide-open sharpness, and much better contrast and colours (but then, the 20mm is only $300 or so).

In short, from what i've read reviews and such, if you can afford the 21mm ZE Zeiss, go for it, I wish I could. 1mm longer probably won't make too much difference to your framing I hope...