Which would give better image quality Canon 15-85mm f/3.5-5.6 or Tamron 17-50mm f/2.8?

The canon is a much better lens ergonomically but is it better optically? Is there a difference between in and the kit lens if you ignore focal length? Same aperture whcih sucks but are the elements in the lens enhanced? If you took two pictures with the came settings with both lenses would they look... which ones better?