I own the Sigma 24-70 2.8 HSM, the actual version of the lens you posted initially and I'm very satisfied.
I had the opportunity to test the lens you are talking about from a guy in a workshop so for me the main difference is the F/2.8-4 is slower to focus and makes too much noise but has a macro ratio that is quiet interesting.
I looked a lot before buying my Sigma and compared with the 28-70 Tamron. Optically the Sigma has some issues in FX but since I'm using it on a D7000 the lens is quiet sharp. And I do miss the 24mm on the wide side, since I replaced a DX 16-85 Nikkon.
Besides, as I own a Sigma 10-20 with a Lee 82mm filter system, I wanted to keep my set as simple as possible.
I guess both lenses are good options when compared to the high cost / pro 24-70 2.8 Nikon, optically they are very good in DX. There is also a new Tamron 24-70 2.8 with VC (the first lens of this type to have this feature) but it's more expensive but all the current reviews have been great.