I used to own the Sigma 150mm and sold it for the Tamron 180mm.
The main reason I sold it was not cos of sharpness or whatsoever, but the Sigma 150mm does not live up to the focal length of 150mm. I found out this when I was out shooting with another friend who was using the 180mm on a 1.3crop factor and I was using Sigma 150 on a 1.6crop factor. At the same distance (about 2-3m away), he was able to get a significantly more close up view than I did (180x1.3=234, 150x1.6=240), which should not have been the case.
I then did further tests comparing with my 70-200 lens set at 150mm, and zoom lens was able to give a more close up view at about 2m away. The 150mm is more like a 130mm.
Thus, I sold it and gotten the Tamron 180mm. Its longer focal length enables me to stand further away from the skittish bugs that I'm shooting, with a wonderfully rendered bokeh
My recommendation is to go for the Tammy, if the focal length is indeed important to you. If not, the sigma would be a good alternative if you're worried about the weight and size and slow focusing of the tammy.