In my personal experience:
Sigma 85: better wide open - sharper, much better corner sharpness, better colour and contrast.
Sigma 50: better closer focussing distance, soft corners - but can be used for creative effect, i like the dreaminess quality to the lens.
Both lenses have buttery smooth bokeh, almost identical build and the differences between the two in centre sharpness terms is pretty negligible by 2 - 2.8
In my own experience, AF is more accurate on the 85, especially at closer focussing distances wide open, although the AF on the 50 might be a smidgen faster, there isn't much in it though.
Personally, now I've done the change i actually think the 35 1.4, 85mm 1.4 and 135 f/2 lenses are a better combo than the 50 and 85.
I think the Sigma 50 is a great lens, the 85 is better (imo) but the difference between the two quite small optically and in focal length terms. there might not be enough difference to justify having the two so opt for whichever one works for you.
This comparison from the digital picture is representative of my own experience:
http://www.the-digital-picture.com/...LensComp=473&CameraComp=0&FLIComp=0&APIComp=0