Sigma continues to impress me with their updated versions of pro and semi-pro lense for D-SLRs. They provide excellent value with pricing below the Nikon and Canon versions while not sacrificing image quality. Their new 24-70mm f/2.8 full frame HSM (same as USM or AF-S) is no exception...
[David] Sigma 24-70, Another "Pro Value" Champ from Sigma
Tony -- I don't think I ever shot with Sigma's previous version, so I don't have anything to compare it with. Spec-wise, Sigma raves about improved coatings on the new version, which should help, but of course its easy to say stuff in specs.
Frankly, I found the Nikon 24-70 pretty susceptible to flare (surprisingly so) when I reviewed it, but maybe my expectations were too high. You might need to rent one, or take advantage of the pretty liberal return policy at B&H to try the Sigma?