The constant f/4 on the 17-40mm isn’t really an advantage, considering it only goes to 40mm. The 15-85mm is actually faster at the wide end, and goes all the way to 85mm (f/5.6). And the 17-40mm isn’t very sharp wide open. Not the one that I used to own. Had to stop it down to about f/8, and it still was far worse than my Tamron 28-75mm at the corresponding focal lengths. It’s its FF-compatible ultra wide angle abilities you would purchase it for. I’ve owned both, and the 15-85mm is the better choice.