From what I recall reading the Zeiss 50mm f/2.0 lens is better optically, though it's a slower lens. If I was buying a 50mm I'd probably go with the Sigma 50 f/1.4 Art lens since I hear raves reviews of it. Autofocus at f/1.4 should be useful as well. I guess Zeiss may have a better look aesthetically while Sigma has a sharper look.
I see Zeiss has some discounts of which some end today. I've been wanting to get the Zeiss 100mm f/2.0 for awhile now since I've heard such great things about it; I've also used it from rentals a couple times. Though, I may start investing in Cinema lenses soonish rather than high quality manual focus still lenses.
I've been debating between the Zeiss 50 1.4 and the Zeiss 50 2.0 actually. The 2.0 can do macro, so thats a big plus... and @ 1.4 the lens likely gets a bit softer wide open.
I have the 85 1.4.. and @ 1.4 the CA isnt great, to the point where I try to shoot at 2.0-2.2 typically (rather that 1.8 or below).... Makes me think I'd likely do the same with the Zeiss 50 1.4....
I have the zeiss 28 2.0 as well, and the CA is not bad @ 2.0... Maybe 50mm 2.0 is the way to go