I like both ZEISS and Sigma.
Because Zeiss is of great quality, well respected, and fully manual. Whereas Sigma is the same but it has auto focus on its side. I love to photograph people and animals, normally you can place me with other portrait photographers and when I photograph people in controlled settings then I would prefer the ZEISS lenses over Sigma since I now have time to make and get the shot. On the other hand I like to take candid portraits which would something like the Sigma Art lenses since they have auto focus and try their very best to compete with ZEISS. Another reason of why I'll take Zeiss is since I want to make videos and movies, their lenses have a larger focus throw at a portion of what I normal cine lens would cost, except for the 85mm f/1.4 Otus that I want at over $3,000. The only other lenses that I want will be a Nikon 24-70 and 70-200 for focal reasons, unless one of the two companies create a fast lens along those focal lengths