How does the Canon 16-35/4 compare with the Zeiss Distagon 21/2.8?

[gigabellone]

I know those lens are much more different than they are alike, but i think they both excel at landscape photography. Is there any IQ difference at comparable focal lenghts and apertures? I would use either one of the lens for landscapes and long exposures, so IS isn't very appealing to me, and i already have the Sigma 35/1.4, which i love, so i don't need the zoom coverage at the longer focal lengths. Going ultra-wide at 16mm would be nice, though, but the faster Zeiss could get me started in night landscapes/starfields photography. Is there any of you that used/tried both?