The new announced Sigma 135mm f/1.8 DG HSM Art lens is a great lens at not very expensive price ($1,399 at Amazon/B&H/Adorama), so compared to Zeiss Milvus 135mm f/2 lens (B&H/Adorama), which one is better ? F-Stoppers has just reviewed these two lenses, the conclusion is that Zeiss Milvus 135mm f/2 lens is much better than Sigma 135mm f/1.8 DG HSM Art lens.
In the end, I would be hard-pressed to tell you should absolutely buy the Zeiss over Sigma, despite the Zeiss being a phenomenal piece of glass. I believe Zeiss veterans will pick their champion without hesitation, mostly because the Big Z fanbase is quite the fanatic, devoted bunch and are attuned to the micro-details that make Zeiss lenses, well, Zeiss lenses. But anyone who simply wants a step up from the Canon 135 2.0L should almost certainly go with the Sigma.