So I know this isn't a very common comparison, but I am looking at buying the Canon 6d as well as a new lens. The way prices are looking, I can get the 6D w/ the 24-105 and a $500 rebate for $2600 while the 6D w/ the Tamron would cost $2800 with a $100 rebate. From experience, what is the optical and performance difference between the f/4 and the f/2.8? Is it worth the extra money?