Ill start by saying that I hate when a thread starts by specifying the brand/level of lens wanted yet the use isnt the main feature of the purchase. I have used both the lenses you want, and the 17-40, and 24-70 on crop and full frame. I dislike all of them on crop. They are either way to long or have too short a range. If i was buying for crop there is no way I wouldnt buy the 17-55/2.8 IS. Good focal length rated by lots of people to be better than the two other Ls. Plus 2.8 and IS.