The 2.8-IS mark1 is the worst 70-200 lens Canon has ever made, it is not sharp, and I tried over 20 copies.
So I have been using my non-IS version, witch is much better.
Only recently I bought a mark2 version, and that is an excelent lens, but that comes at a price.
The Tamron seems to be a very good lens too, not as good as the mark2, but better than the other two.
I can not say for myself, because I never worked with the Tamron.
So, if money is an issue, buy the non IS, great money for value.
If you want IS, buy the Tamron.
If you want the best, buy the mark2.
And you also can consider the F4 versions....