A summing-up image from a recent article I wrote about 24-70 mm f/2.8 lenses for Nikon and Canon. Besides the brand native lenses I included Tamron 24-70/2.8 VC and the Sigma 24-70/2.8. Appointing an overall winner was not easy as each lens had its weak and strong points. However, seen as a whole, the native lenses did ever so slightly perform better than the third-party offerings, albeit margins were narrow. Factor price into the equation and the ranking might be different too.
I might try to do an effort and translate that article written in Norwegian into (pseudo-)English?