I have the non OS version of the Sigma 17-70mm lens. I like the edge sharpness compared to the Canon 17-85mm which is really quite bad. Haven't tried the Tamron but I saw a review that rated it similar to the Canon, so I suggest you test first before deciding.
You're of great help! Now i have the second problem. To get the OS or the version without...
Like what you said, i came across plenty of reviews saying the old 17-70 without OS is sharp. So my question is, have you tried and compared the one with OS and the one without? If you have, then is the sharpness issue a problem in the new 17-70 with OS?