I had a hard time myself selecting which zoom to buy, comparing tamron, sigma and canons together, in the end I went and got the canon. WHY?
I tried the sigma and the tamron in the field and returned them immediately, They do a lot of hunting, the sharpness is acceptable but not all that great.
I got the canon 24-70mm 2.8 L from ebay for about $1000 and you just cant go wrong. You'll wonder why you didnt get it sooner. And the bokeh is amazing on 2.8
I have a few video's I made using the Canon & and the 24-70mm at 2.8
here they are
http://www.youtube.com/watch?v=T_x87aT4WZY
there are more there with the canon 35mm 1.4 L