It's true that they (17TSE vs 15 ZE) shouldn't really be compared, but it's tempting as they are the kings of the ultrawide Canon-mount hill in regards to image quality. I have also compared Canon 14L II vs. Samyang 14 vs Zeiss 15mm in real-world scenarios and in that comparison, sharpness wasn't a factor to think about, they were all rather close in sharpness. The Canon had excessive CA, the Samyang had horrible distortion, and the Zeiss had a horrible pricetag :-) They were all pretty much similiar in sharpness, but Zeiss has to get the prize for ultimate color rendition and all-round great quality. No CA, no distortion, no bad sharpness. The same can actually be said about the 17 TS-E, but it's an 17mm f/4. The TS-E has minimal vignetting, while the Zeiss is a 2.8, it's excessive vignetting actually results in much darker corners than the 17 TS-E.
I have six ZE Primes, 21 to 135 as well as the 17mm TSE. Everytime I look at this comparison I keep wanting the 15mm ZE, but the versatility of the 17 makes it hard to sell. I also want a 24mm TSE as well. Though part of me thinks that if Canon releases updates to the 45 and 90mm TSE, I might have to give them serious consideration over the Zeiss lenses. Though I would likely keep the 21mm and the 35mm 1.4.
