What's the confusion? The Tamron 70-300mm at 300mm delivers pretty bad IQ across the frame, better in the center than the edges, but not very good in the center either. Compare it to the 70-200mm f/4L at 200mm (
link), for example, to see what a sharper center would look like (and there are many even sharper lenses, I just picked one in the approximate same price range). The Canon is not much better at the long end, and as Bryan points out, has better AF, less vignetting, and less distortion.
The bottom line is that you're probably better off getting the 70-200mm f/4L instead of either the Canon or Tamron 70-300mm lenses, from an IQ standpoint.