I used to have the Sigma until it was stolen. When it worked, it was awesome. I loved the size and feel of it, the pictures were sharp, and CA was minimal. HOWEVER...it missed the focus at least 1/3 of the time. The inconsistency was consistent in that when it missed, it was because it was focusing in the lower left corner (landscape).
When it was stolen I thought long and hard about replacing it with another Sigma and for the price, I settled for a Canon. I had rented the canon before and I found a new one for sale for under $300. It wasn't worth the extra $200 for the Sigma IMHO. I may upgrade if I feel that I can get a Sigma that works consistently or I win the lottery, but for now I am quite happy with the Canon 50 1.4.