I was sooo torn whether to buy the Zeiss 15mm. I have the 21mm 2.8 and know that the 15mm would be just as incredible...but after much internal argument...I bought the Canon 17mm TSE f/4 II. I did this for three reasons.
1. Control of the imaging with the T&S mechanisms
2. Conversion to 24mm TSE with 1.4X III (very good quality) I already owned the extender.
3. Cost (which was the lowest of my concerns, but we all like to keep a little money in our pockets).
If I could have both lenses I would.... The Zeiss is more of a "hand-portable lens" and has f/2.8..(but that is not so much a concern for me with a wide angle).
Both are incredible tools.
And things are evolving even further for me toward Canon, I am surprised to say...because I do always think out of the box. I own the Zeiss 21mm f/2.8 ZE which I have currently posted to sell on Ebay. One of the main reasons that I had that lens was because of the weak performance of my Canon 16-35mm f/2.8L II. The images from the Canon did not compare to the quality from the Zeiss....but I have recently sold my Canon 16-35mm f/2.8L II and purchased the new Canon 16-35mm f/4L IS. This lens is so sharp that I have decided to sell my Zeiss 21mm as I have great IQ, AF, versatility (in focal lengths) and portability with the new Canon UWA zoom.
This new zoom give me this great IQ, AF and portability whilst my 17mm TSE gives me more control and precision when I need it. With the Canon 16-35mm f/4L IS, the 17mm f/4 TSE and the 1.4 III I feel I have a lot of UAW lens power in my wide angle focal lengths. I also have a Sigma 35mm f/1.4 Art to round it out when I really want AF and lower DOF for environmental portraiture...
but.... I am not leaving the incredible Zeiss glass out of my quiver....LOL!!!...I plan on buying the Zeiss 135mm f/2 ZE as soon as I sell my Zeiss 21mm. Dustin's review just blew me away, so I just have to have that piece of glass in my cache at the longer end for low DOF when I am not using my zooms.
The Zeiss 15mm is a droolfest, though...no doubt!!!!!
Can't go wrong with that choice.
I attached an image that I shot with my 5DIII and the 17mm TSE in almost total darkness..I needed a flashlight to set up the T&S. LOL.