Well as an owner of the Tamron you mentioned I can tell you that I've been very happy with it.
Pro's:-
Sharp with decent contrast.
Light as far as lenses this size go, great for walkabout duties.
Constant f/2.8 through the range, this would probably be helpful when being used for portraits aswell.
Con's:-
Build quality isn't as good as the Sigma 24-70 f/2.8
No USM, so the focus motor can be a bit noisy, but no too bad.
Only 28mm at the wide end, not the widest on a 1.5/1.6 crop factor camera... but then again you have the kit lens for wide duties.
I've no knowledge on the Sigma 17-70 f2.8-4.5 DC Macro you mention but I personally did alot of online research compairing the Tamron 28-75 f/2.8 and the Sigma 24-70 f/2.8 before buying and came to the conclusion that the Tamron just edge's it in the picture quality stakes (but not by alot) and the Sigma wins in the build quality stakes at the cost of extra weight.
I went with the Tamron because I wanted a light walkabout lens with very good picture quality at a decent price... you pay's your money, you take's your choice.