I am considering between the Canon 100-400L IS and the Tamron 200-500 before taking part in a birding class. Apart from the price difference, the reviews I got seem to suggest that the Canon is more usable whilst the Tamron reaches further. Has anyone here used both? Can you tell me: 1. How much usability will I gain with the Canon, and how much further can the Tamron reach? Given that I will use tripod for the birding trips, while still expect to handhold for other purposes. 2. How often does the IS in the Canon help (not to miss a picture)? and how often do you use that last 100mm (400-500) in tamron? 3. Is the Tamron good for sport? i.e. for a football match. By the way, I'm using a 20D. Thank you, Quynh.