Ultra Wide Zeiss 21mm or 24mm Tilt/Shift lens? (Nikon/Canon)

Reader Ron L asks:

I really want a good wide angle lens for nature, landscapes and some indoor use (product-type photography). I shoot with a Nikon D700 and have a 17-35 for general wide angle use. I could afford to buy one of these lenses. Can you give me your opinion of which one you would recommend? I know I haven't given you much info here on how I shoot but perhaps I have given you enough for you to hazard an opinion.

I could spend many pages answering this question, but I’ve given it some reasonable discussion which I hope will be of assistance. See my answer in Ultra Wide Zeiss 21mm or 24mm Tilt/Shift lens?.