Price has to pay a factor in everyone's decision. Everything I've read says the image quality out of the two is so similar that it's hard to choose.
Well, for me. It's easy. The Sigma is half the price of the 85L and focuses faster.
1.4 vs. 1.2, that's the only thing that the Canon has going for it.
Both are great lenses, but for the price of the 85L you could buy the 100L and 85/1.4 Sigma.