The sigma not only has ring usm, but also OS(I'd just consider this a small perk, doesn't matter if you're shooting sports but for indoor static shots it's good).
The tamron autofocus is fucking useless, didn't find it very accurate(although having not had a chance to use the sigma I can't comment on this) and it's the slowest thing I've ever used, not to mention being noiser than a fucking jet.
Given what I've read on the internet the sigma is "nearly as sharp as the 17-55" but I'd want to see real tests to make sure. Also not sure how it handles in other respects.
If you were JUST shooting video, sure I'd say get the tamron, it's dirt cheap and you don't need autofocus, but you did say photos, you need to ask yourself, how much do I need a good autofocus system and OS? is it worth the extra couple hundred? or can you live with a poor autofocus/manual focus and save yourself a lot of money if you're not shooting many photos?