Canon EF-S 15-85mm f/3.5-5.6 IS USM vs. Sigma 17-50mm F2.8 EX DC OS HSM

I have a Canon 7D and I decided on the Sigma 17-50mm F2.8 EX DC OS HSM lens as a good walk around lens. A was also looking at the Canon EF-S 15-85mm f/3.5-5.6 IS USM. It has a great zoom range. Obviously the Sigma is faster with 2.8 all the way through.

How does the IQ compare between these 2 lenses? I can see where the Sigma has the advantage under less light conditions as well as the lower price point.

Thanks for any input.
Mike