I own a 17-40 l have had it for couple of years now and I can say that it's a fantastic lens very sharp and I tend to use it a lot even on the 7d I now own a 5d mark 2 and it is exceptionally good I am not sure about the 15 -85 never used one before but I would buy the 17-40 for the following reasons a L lens so the built quality will be much better than the 15-85 (metal vs plastic) so more weather sealing I find this extremely useful for landscapes. The 17-40 l lens does not extend when zoomed in keeping it nice and small but the 15-85 extends and can be a bit troublesome to use and does not come with a hood. They are just some reasons.