I would also go for the 15-85; the 17-40 just does not make sense on a APS-C camera, you pay too much, you carry arround too much weight, but you gain nothing.
IQ of the 15-85 is very good accros the (very usefull) range.
As Marsu says, the 17-40 isn't much more than the 15-85 in cost and is actually lighter, so not sure what you mean by 'you carry around too much weight' unless you mean by using the 17-40 as a walkaround means you need to use it in combination with another lens due to it's limited range. I got the 17-40 as a weathersealed lens to complement my 7D. Used on APS-C it will also have less vignetting, possibly less distortion at a given focal length than the 15-85 and a constant aperture. And you could use it on a FF camera if you chose to upgrade.
Having said all that, I concede that the 15-85 is a very nice range for the crop sensor, and a travel light one lens solution. The IS is also something the 17-40 lacks. I've never used it myself so I can't say how much sharper than the 17-40 it is, but one thing to consider is that it is a lens that MAY suffer from centering defects to a greater degree than normal, I'm fairly sure there have been several threads on this forum complaining of corner softness and uneven vignetting, and Photozone also mentioned decentering in their review (not sure how many copies they would have tested). Of course, I'm sure you can get bad copies of almost any lens.