Tamron 28-75 2.8 vs sigma 24-70 2.8?

The 28-70 is a bit cheaper, but what's the difference, I have heard great things about both
The Nikon 24-70 is out of my range.

Usually I prefer Sigma over Tamaron, cuz Sigma had better build quality and performance.

you can find comparison in these site.
http://www.ishootshows.com/2010/10/18/comparison-review-tamron-28-75mm-vs-sigma-24-70mm-f2-8/

I think you can't go wrong here, the only problem I may see with the Tamron is the weird focal 28mm, if this doesn't bother you then you'll get big bang out of this lens. $500

If it bother you because you prefer to have an extra 4mm for landscape then the Sigma 24-70mm f/2, 8 $800 is the best option (my opinion), better than than the new Tamron 24-70mm f/2, 8 vc who cost $1300.

But if you chose the Sigma you need to take consideration of the "filter thread", with 82mm if you want to buy filter like a circular polarized filter, neutral density filter or graduated filter it will cost you more money than the Tamron 28-75mm f/2, 8 who has a 67mm filter thread.