This in-depth head to head video comparison by photographer Matthew Gore (from lightandmatter) examines the differences between the Tamron 24-70 f/2.8 VC USD and the or the Canon 24-70 f/2.8L II USM. The review covers build quality, AF, Image stabilization and of course image quality (both sharpness and resolution).
So which one is the clear winner? Gore concludes that both lenses did very well with a small advantage in sharpness for the Canon but on the other hand the Tamron does have image stabilization which the Canon lacks. At the end of the day for many people it all boils down to money and while you can find the Canon 24-70 f/2.8L II USM for around $2300 (currently at B&H), the Tamron 24-70 f/2.8 VC USD cost significantly less (currently going $1300 at B&H).
You can find many more previews and reviews on our Photography review section here on LensVid.