i have no experience of the 105DC.
but the Sigma has lovely creamy bokeh and is sharp from 1.4 and very very sharp from f2.
i'm at work at the mo so i don't have access to my image library but as an idea:
this is a shot i took at christmas, 1600 iso f1.4
and here is a 100% crop - absolutely no post processing/noise reduction or sharpening, in good light at lower iso i have photos which are notably sharper than this also.
when i look at the flickr group for the 105DC whilst it is undoubtedly a very fine lens, the bokeh does seem to be a little bit "busier" than the Sigma 85, but that could be down to lots of things but certainly if you are wanting a bokeh monster then the Sigma 85 will fit the bill, whether there is enough focal length difference to warrant having the two, i'm not sure.
Looking at the crops on the-digital-picture it seems to be that both lenses are about equal in sharpness across the frame at similar apertures. AF speed of the Sigma is good, not rocket fast like the 70-200 or 135/f2 but not bad at all, certainly keeps up with my little girl most of the time.
http://www.the-digital-picture.com/...meraComp=614&SampleComp=0&FLIComp=0&APIComp=0
i'm really pleased with the performance of sigma 85 - it's certainly one of my favourites (my absolute all time fave is my 70-200, just because it is incredibly versatile), i would actually place the sigma 85 over the canon 135 f2 for bokehliciousness and it's the lens i turn to now for narrow dof stuff. it's an excellent lens.