From what I have seen of the Sigma 24-35, it will offer superior FX corner/edge performance at small apertures compared to the Sigma 24-70, Tamron 24-70 and the old Nikon 24-70. I have not seen enough samples from the Tokina 24-70 to draw a conclusion there. There is actually not a wealth of good samples available for the Sigma 24-35 but from what I have been able to find, the corners are quite strong.
The new Nikon 24-70 VR will likely be as good or better in the corners based on what I have seen, although at a significantly higher price