+1 on SteveU's advice.

Though, I still think the 15-85 is better than the 17-55, overall. The range is greater. The 17-55 is soft at 2.8, and once you stop it down to, say, f/4 (if you want sharper images), you'll find that the 15-85 still is sharper. It's sharper over the whole range of the 17-55.

Decisions, decisions.