I agree the 21mm is better, but I do love the 18mm Zeiss on my D700 (full frame). However I knew what I was getting into when I bought it. Getting below 20mm (ish) on 35mm film or full frame digital cameras, I personally have never found the "perfect" lens, be it fixed prime of zoom. You always seem to give up something.
Years ago I talked to a profession optician, and not the eyeglass kind, who worked designing lenses, and if memory servers me correctly, the "magic line" or whatever you want to call it is around the 19 to 21mm range on 35mm cameras ( and by default, full frame today). Beyond that point, the difficulties in making a lens where everything performs "perfectly" does not just double in difficulty, it jumps up on a exponential scale in difficulty, or in plain language, a factor of ten or more.
So to get a "perfect" 15 to 18mm range lens - however you want to define that - you would be looking at a lens that costs far beyond the $1,300.oo range, no matter who makes it. How many full frame lenses below 18mm for Nikon or Canon are there out there right now? Or better yet, how many of them are NOT fisheyes and NOT for APC format sensors only? I haven't seen or used it, but I know Zeiss has a 15mm for Nikon and Canon, but that lens is about $3,000 if memory serves me correctly.
So the way I see, using one myself, the 18mm is a specialty lens. If you want sharp, low distortion, etc, go the 21mm Zeiss (which is about $,900 I think?)
but otherwise, in 18mm, it si a stand alone product, and I enjoy mine for what it is