I'm trying to decide between these two telephoto zoom lenses, the Canon 70-300L or the Tamron 70-300 SP Di VC. Has anyone done a comparison on these two? I have read they are both very sharp but there is a huge difference in price. I also wonder about the Tamron's auto focus speed. Any input would be appreciated. I'll be using a Canon 7d body. I take outdoor and sports photos.