I also have the 70-200 2.8 IS
and it is a good lens.
I am a little spoilt as i have an above average 100-400L that simply takes exceptionally sharp images and the quality of the two is very high and I am still surprised by there sharpness.
The 70-200 is only slightly better that my 100-400L in the 100-200 range but i am keeping my 400 for its great sharpness at 400mm.
I had read from several reviews prior to getting the 70-200 that the Sigma is also very good for its price.
Some reviews from people who have used both stated they are getting similar quality to the 70-200 2.8 IS
Its not as good as there 120-300 2.8 which from several pro photag's is their lens of choice over the Canon 300 2.8 .
Having seen some pics from this awesome lens i can see why!!
If you can afford the extra 200 Euro i'd go Canon as well as you will be pretty sure of build etc.
Whethter you want the IS
is up to you but it does cost a fair bit more.