Oh my.. The competition in the 24-70mm f/2.8 zoom lens segment seems to be extremely busy at the moment. Sigma should have an 24-70mm ART lens announced any day now, and now Tamron is rumoured to announce a new 24-70mm G2 this week too, according to canonrumours.com. (also here)
Interesting enough, both are rumoured to start shipping with an initial price tag of 1299 USD.
I guess I will have to wait for some real life test and reviews before deciding then. I used the Tamron 17-50mm f/2.8 VC a lot on my 60D, and it was an excellent lens. If the new Tamron proves to live up to this, I might be interested.
Additionally, Sigma released some MTF charts that people "wise" in such things seems a bit disappointed about. I never decide on lenses based on MTF charts, so my advice: Wait for some real reviews by trusted sources.