I played with mine a little today. Did a few comparison shots with the Tamron, but not under ideal conditions. A dark and rainy day. When I first started shooting, the Sigma seemed to be the clear winner. I've had problems with the Tamron focus hunting. Then I remembered I had my UV filter on the Tamron, so I removed it to compare both without filters, and the Tamron wasn't focus hunting anymore. So all these months I've had that lens with the filter on and have been frustrated with the focus hunting, not realizing it was my filter causing it.
The Tamron seems a little sharper than the Sigma. The Sigma gives a nicer background. The Tamron shoots warmer, which I like.
The Sigma has a panning mode. The Tamron doesn't. I think the Sigma can also be used with a TC, while the Tamron can't. Haven't looked into this for sure yet.
I prefer the direction of the focus ring on the Sigma. I prefer the tripod foot of the Tamron. I usually carry the camera/lens combo by the tripod foot when I'm walking, and the larger foot on the Sigma lets me wrap my whole hand around it so I don't fear dropping it. Can't get quite as good a grip on the smaller Sigma foot. Haven't tried swapping them yet to see if one will fit the other.
Haven't quite made up my mind which one to keep. I want to try shooting under better conditions before I decide. I was hoping there would be a clear winner, but there isn't.