Can I ask what sort of lens do you currently have?
I have a friend who's looking for really wide for landscape, same situation you're in. He's already got the 24-70 AF-S, 50/1.8, 70-300 VR and 85/1.4 .. my advice would be to simply upgrade the D90 (his) to a D700 and issue solved.
The 21 CZ is simply amazing, can't say anything more about it. I wanted to go for it, or the 18mm, but saw the 21 to be better. The price had kept me off either, hence why I went for the Voigtlander 20mm.
21mm is the better lens in terms of IQ for MTF, CA, and distortion.
the main problem with these zeiss lenses is the levels of vignetting while the 18mm has a complex mustache distortion.