I have had both lenses. That price for a sigma is a great price, I paid around 450 for mine.
The sigma has better build, and image stab, and focus,is quick. I actually found it to be similar in iq to the kit lens.
The tamron was very sharp, and has constant f/2.8. Focus is noisey, slow, and tends to hunt in low light. I actually really liked it, but the range 50 mm at the long end was to short for me.
I ended up buying a canon 15-85, more expensive, but a great lens, iq, imag stab, range and build.
On the two choices you were asking, even though the sigma is a very tempting price, and many people love it, I would buy the tamron used, you can get em here all day for around $325.