My bad, I didn't realize that you were talking about the HSM version of that lens. I took a look at both reviews that you posted and, while I have not personally used that lens, there are too many negative reviews there for my liking. It seems that many of the reviewers mention blurry or soft focus. Granted there are reviews that mention the same of the Tamron lens too.
My experience with the Tamron has been a positive one. I seem to have a sharp copy and have had no issues with quality (either image or build). My only complaint is that the AF system is noisy, though I find it to be fast and accurate. And though this is a relatively minor point, I love the Tamron lens cap. Even if you get the Sigma, do yourself a favour and go buy the Tamron lens cap as it is easier to use with the hood on.
And to make your decision more difficult, Tamron just released a VC (vibration compensation) version of the 17-50mm:
Tamron launches stabilized 17-50mm F2.8 zoom: Digital Photography Review.