I did a comparison test of 24-70/2.8 lenses from Canon, Nikon, Sigma, and Tamron that was published in a leading Norwegian photo magazine a few months ago. Suffice it to say that prices varied more between these lenses than did their quality under actual field use. As each had areas in which it excelled, declaring an outright "winner" is tricky and indeed a rather futile exercise, as the individual user preferences and requirements must be factored into the overall equation. One probably could live quite happily with whichever of these lenses that fitted your camera without totally emptying your wallet. If peak AF performance should be the decisive criterion, go with the native lens for your camera system, however.
(and yes, Sten's Tamron VC was included in these tests)