17-55 goes wider and has the faster max-aperture, and is about the same price as the 24-105 (give or take 10%).
The only reason you should consider the 24-105 would be a) you really need weather sealing, or b) you really need the 55-105 length (and the 15-85 isn't fast enough), can't afford another tele-zoom, and don't need 17-23 (like if you've got a 10-22 as well).

Coupled with the 85/1.8 or 100/2.0, i'm calling the 17-55 the go.