The Canon 70-300 (non L) is very nice, the sharpness drops off beyond 200mm, but I never really found it a problem. My biggest gripe was the AF when tracking fast objects like aircraft, searching back and forth at critical times, which isn't so much of an issue in slower moving subjects. I still got some great high speed target shots with the lens, but grew frustrated at times.

I upgraded to the 70-300L before Xmas, which is a real step up, but I'd certainly give the Canon non L a huge thumbs up for the 5 yrs service it gave me - I've never used the Tamron, so can't really offer an opinion on that.