The 17-85 is much softer, has more chromatic aberration, an older IS system. Check this comparison at 50mm, mouse over the image (or the arrow above it) to see the 17-85 results. And this comparison at 24mm really shows quite a big quality difference.


Also, using the flare comparison tool, the 15-85mm comes out ahead. Finally, the distortion test favours the 15-85mm slightly too.


In other words, the 15-85mm improves every optical aspect of the lens over the 17-85mm.