Decide on the focal length that would better serve your needs and then go from there.
The Canon Ls are better portrait lenses than the Sigma Arts IMO. For portraits, you don't want ultimate sharpness. You want great rendering quality. Both the 50L and 85L will give you good rendering quality in spades, though both are sort of prima donna lenses (they will give you some stunning results but are difficult to use). The Sigmas are a little sterile if you ask me, but to each their own. Unfortunately, photography seems to be getting about looking at sharpness figures and not actual photos.
It's easier to decrease sharpness than increase what isn't there...
And sterile and canon has better "rendering quality" could you explain what you mean?
The 200 f2 is a lot like the 50 art, no distortion great color and contrast which combine with no ca makes the subject pop. Low vignetting and great corner sharpness makes it VERY useful for off center composition.
I'm sorry but I simply disagree with you. I think the 50 art has a great look to the images and that's not something magic, it's just very very nicely done and very well corrected.