Someone mentioned that the Canon 85mm 1.8 has more unreliable autofocus. I am just posting to say that I have not found that to be the case. The Canon 85mm 1.8 has the best autofocus on any possible lens that I have tried, better for sure than the 70-200 II, for instance. It's never unreliable unless user error causes it to be so. (I.e., you mis-place your focusing points, such as accidentally beginning to press the button before the AF points are actually on your desired subject.)
The Sigma 85mm f/1.4 has slightly unreliable autofocus, I have found, but good enough that I can still rely on it at a basketball game (not 100% reliable). But Sigma's image quality at f/2.0 is far better than the Canon just because of the CA problem (chromatic aberration) that the Canon has.
With the Canon, you will get purple reflections from the out of focus stadium lights reflecting off of glasses, for example (ruining an otherwise good dunk photo, for example) if you shoot below f/2.8. So basically you are wasting more than one stop of the len's potential in order to avoid CA.
I don't have to worry at all about CA with the Sigma lens at f/2.0.
CA is the problem with the Canon, but its auto focus is as close to perfect as possible on all of my copies and every else's that I know. So I had to chime in and say something.
Yes, I have had disappointing focus experiences myself, but it has always been user error of my own like getting too eager and starting to lock focus before a subject is close enough to even register on the AF sensor. If I make a mistake like that, the lens will do a perfect job of tracking focus on the background. The way to fix that mistake is to take my finger completely off the shutter and re-acquire focus on the proper subject. So if you are experiencing any focusing problems with the 85mm 1.8, just wait a little bit. AF takes technique from the photographer as well as capability and accuracy from the lens. I am still learning and improving every day.