if you're looking at the VC one, the reviews I've read peg the new Sigma 17-50 OS HSM as a better choice, as it closes in on 17-55 territory. I've also read that the non-VC version of the Tamron is sharper than its VC cousin.
I personally have the Sigma 18-50 non-OS, and I sometimes find myself looking at the newer 17-50. But I'm so happy with the sharpness of my Sigma that I think it'd be too much of a hassle just to add OS
Although, HSM would be nice
