Well, I love my Tamron 28-75. I'm sure the Sigma 24-70 is a good lens too, but most of the user reviews I've read (and trust me, I've read MANY) seemed to favor the Tamron. Of course, I'd take any single-lens to single-lens comparison with a grain of salt, since a bad copy on either side would make the comparison pointless.
I was debating over the Tamron 28-75 and the 17-50. I know the 17-50 is a more practical walk-around lens on my XTi, but I figured that I'd probably want a super-wide (perhaps a Canon 10-22 or Tokina 12-24) and a telephoto (70-200) one day and the 28-75 range fit right in the middle without much gaps. Also, the 28-75 is cheaper and will work on a full frame camera (not much of a concern though). I bought the XTi without a kit lens, then bought the 50 F/1.8 ($75), then the Tamron 28-75 ($330), then the kit lens USED. Yes, I fully intended to get the kit lens once I felt I needed the wide range (it's a decent lens at small apertures [large aperture values]), but knew that I could get a used one for pretty cheap since people were replacing it with better lenses.
The fact that these Tamron lenses have no IS isn't that big of a deal to me, especially for landscape shots. I'd be more concerned to get IS on a telephoto, since my shaky hands will have a more dramatic effect.