I'd say that it really boil down to individual lens' performance. I have had that Tamron 17-50 non-VC lens for over 3 years and I'm extremely happy with it. My husband bought a Sigma 17-70 (the older version without OS) a couple of years ago and equally happy with it. I've done some comperations some time ago and found that between 17-35mm they're almost as good, at 35-50mm mine is
slightly better in my opinion, although my husband would say that I'm imagining things. Above 50mm his lens performs vastly better than mine though...