The Tamron is way better if you find a good copy.
I used to have one in Canon mount and it was an excellent zoom.
The AF is slow, the built construction is nothing special, but the optics are wonderful. You can't ask for more in this price range.
The lens is also compatible with "full-frame" sensors but, if you think you'll just use it on an APS-C body, the Tamron 17-50mm is also a good wider alternative.