I've owned the 85 L II and sold it for the Sigma. My advice is to buy the Sigma. I'm not sure there is any real difference in image quality. Focusing on the Sigma is so much faster. Also, with a filter on the Sigma, it's more sealed than the Canon if you are worried about dust.
So, for less than half the cost you get faster AF, better build quality, tie image quality, but lose half a stop.
With high ISO performance available ( with e.g. 5dii, 5diii, 1Dsiii, 1Dx) , the effect of large aperture lenses (i.e. half to 1 stop difference of light allowance) has diminished or even been eradicated to a large extent.
That leaves Quantity and Quality of OOF Blur as still one of the major advantages large aperture Primes provide (apart from sharpness etc).
While I have not worked with the Siggy 1.4, I have compared images of it on TDP: and the difference is pretty clear between the 2. If you go here http://www.the-digital-picture.com/Reviews/Sigma-85mm-f-1.4-EX-DG-HSM-Lens-Review.aspx
about 1/3 of the way down you see a Picture of a Girl taken at different apertures with the 85L and the Siggy 1.4 against a forest/ green background next to a Black cast iron Fence.
I find the Bokeh of the 85L to be better.... but your mileage may vary since Boken is subjective... however
Even the Quantity
of OOF blur is more with the 85L at the same
aperture. e.g. Compare the tree trunk blur behind the girl with both lenses at f1.4, the 85L melts it so much more... in fact the 85L at F2 has as much subjective blur as the Siggy does at F1.4... this is where the 85 L shines. I don't know of many lenses that can melt away the background like the 85L.... but you Pay for it