EISA recently named the Sigma 135mm f/1.8 Art as the best DSLR lens for 2017-2018, and based on my experience with this lens, I have to agree. It is a fantastic lens and performs at a very high level, with incredible detail rendering capabilities and beautiful bokeh. For the longest time, the 135mm focal length had a little bit of a gap that really needed to be filled. Canon has their version, which although is a great performer, it just can't match the likes of the Zeiss 135mm f/2. However, due to having autofocus, many tend to choose the Canon version over the Zeiss. With the release of the Sigma version, it would seem that one may no longer need to compromise.
In my video above, I demonstrate how the differences in performance between the Zeiss and Sigma are difficult to tell apart. Both render detail at a very high level and are equally as sharp when shot wide open. The bokeh that both lenses produce are seemingly identical and I genuinely cannot tell any difference between the two.
Having said that, the Zeiss may still offer a very compelling reason to pick it over the Sigma hidden up its sleeve. The colors from the Zeiss are far more accurate and it's able to pick up many more tones and produces a warmer and more pleasing image. Looking at the example below, you can see the difference between both images. The Zeiss is far more effective in capturing skin tones and is more detailed and accurate. The Sigma produces a much colder image, and not only that, it's missing many of the skin tones the Zeiss is picking up.
All examples below were shot with the same camera and are being demonstrated with the same white balance.
Even once the white balance has been adjusted to compensate for the colder image from the Sigma, The Zeiss is still far more detailed in the skin tones and picks up more colors.
Check out the video for a more detailed comparison between both lenses.