No prior experience with the Sigma, but the Tamron is very sharp, even wide open.
However, what you have to note is 28mm may not be your cup of tea when it comes to wide angles, because it's really not wide at all.
On a digital crop, you may want to be looking for something like the Sigma 17-70 f/2.8-4.5. It's not a f/2.8 constant zoom, but it gives you the focal lengths you are looking for at a small aperture compromise. The 17-70 can give you quite a decent magnification of 1:2.3, and so, performs quite decent close up functions.