The new Tamron with vibration reduction has tempted me more than once. Try and shoot them side by Side. I tried this with the 28-70 and compared to my 35-70. My lens had better contrast and sharpness on a quick test in a camera store so I didn't 'upgrade'.
"Cameras and lenses are simply tools to place our unique vision on film. Concentrate on equipment and you'll take technically good photographs. Concentrate on seeing the light's magic colors and your images will stir the soul." Jack Dykinga