Get Sigma DG HSM Art at B&H Photo.
As this was written, the ZF.2 and ZE Zeiss 135mm f/2 APO-Sonnar are a whopping $623 off. Save yourself a ton of money with the ZF.2 or ZE version closeout prices on a classic that will never go out of style and never have an AF motor fail or misbehave—because it is good 'old manual focusing helicoid.
This comparison used the ZF.2 version. The Zeiss Milvus 135mm f/2 APO-Sonnar is optically identical except for one element with a slightly improved lens coating. The Zeiss 135mm f/2 APO-Sonnar in any garb is an Otus in everything but name and physical build style.
Close range shooting is often a weakness in many lens designs, which intentionally or not “game” the lab testing system by not performing nearly as well at close range as at distance.
This 2-way comparison at a reproduction ratio of about 1:5 pits the Zeiss ZF.2 135mm f/2 APO-Sonnar against the Sigma 135mm f/1.8 DG HSM Art. It assesses sharpness, bokeh, and color correction, also with comments on breathing and focal length. It confirms and complements the results found in the White Flowers shootout using strong red and green tones.
The results are compelling for anyone considering these two lenses. In Guide to Zeiss:
Includes images up to full resolution, with crops from f/2 through f/11. Also contains a 3-frame focus stack at f/8 with the Zeiss 135/2 APO-Sonnar, which is spectacular in its detail and color on an iMac 5K or better.