TS, you have to understand that a flash unit can only do so much.
if the scene is a small indoor setting, you can bounce the flash to light up the area, no issues here.
if the ceilings are too high or the walls are too far away, you really need the fast aperture and or fast aperture + high ISO, in order to capture more (ambient) light.
the 15-85 has great focal length but the variable aperture may be a problem if you want to have a constant setting when you are shooting on site. as you zoom in and out, the changing aperture will affect the overall exposure.
also, the 15-85 will be at f5.0 from 38 to 60mm. this is almost 2 stops slower than a 17-55 at the same range. it will mean bumping another 2 stops of ISO with the 15-85, in order to attain the equivalent exposure as the 17-55.
this are little points which you should consider.