<p>In fairness I have not tried the Tamron, but recently purchased the Canon 100-400L IS II. It isn't cheap, but produces great images. The lens is very well built and weather sealed. Image Stabilization is excellent, and the lens is very sharp. On a 6D you'd get a very useable focal range. On the 7D the "multiplier" would give you a 160-640 focal range with IS.</p>