I happen to own the Sigma, just purchased, (Canon mount--T2i) and have owned the Tammy (actually two, Canon Xsi and Pentax K-r mounts)
The Tammy is a sharp lens, its many reviews laud its optical bang for the buck (given a good copy). I went back to Canon and was torn when shopping for a f2.8 zoom, ultimately opting for the Sigma after much back and forth. That said, I have been nothing short of stunned by my images from my Sigma. HSM, OS, better build...I felt it was worth the extra $$. FTM would have been nice, but I can live without it....
Pentax users have to consider the fact that the in body IS may tilt that decision a bit, but I think the Sigma's in-lens OS may be a stop or so better than the Pentax in body SR. No empirical data, just my observation, plus I've seen similar statements to the same.
Also- interestingly, i recently did a side by side shot comparison of my Sigma 17-50 and my Tamron 28-75 (which is optically similar to the 17-50); same shots, camera body, lighting, FL, AV, etc... I was surprised to find the Sigma rendered colors better, had more "pop", (and was quite sharp even at f2.8!!!) I was quite surprised.
LOL, in the Canon world, the long-revered icon, the 17-55 f/2.8 IS is feeling the pinch (and rightfully so, priced ridiculously at $1100 plus) and you'll notice, at least in Canon mounts, the Sigma is oddly out of stock
Really, you cant go wrong either way. remember, end of day, its the photographer and technique...you can get fantastic images out of either piece of glass. Good Luck!!