Understood.I think you should test it to make your own judgement Rent it and try it. Better having an AF, even if slow in low light (that also depend of the user behind the camera...).
For example, the 85f1.4G is also slow in low light in some situations... I mean, it can't focus at all !! On the D700/D800 I had some problems, less on the D4. Same goes for the 85f1.8G, but better results at the end.
The Sigma can be manually focused if needed so I don't really see any problems after all. Just test the f1.2 canon line in dim conditions
Personally, I wouldn't even consider a 35mm DSLR lens unless it is AF. I sometimes shoot for 10 straight hours and about 7 hours into such a shoot the eye fatigue starts making manual focusing lenses pretty damned difficult ... except Leica M rangefinders for some reason.
I know all about the Canon 85/1.2 in both the first version and so called faster MK-II version. Shot for years with both. Turtle slow AF in low contrast, dim light ... mostly solved by using the Canon STE-2 Transmitter which puts out a good focusing pattern target on the subject.
The Zeiss ZA 50/1.4 doesn't seem to suffer from "Turtle Syndrome", so I'll bide my time and wait for a ZA 35/1.4 if it ever comes.
BTW, if I rent anything to try it'll be the new Leica S45 ASPH ... which is the same FOV as a 35mm in the smaller 135 formats.
- Marc