Since you're replacing your 17-50mm, you can answer which lens you need yourself.

When to choose the 17-55mm:
Do you often use the larger aperture for low-light, indoor, or thin DOF shots?

When to choose the 15-85mm:
Do you often find yourself at 50mm wishing you could go further?
Do you often find yourself at 17mm wishing you could go further?

After you've answered yes to all three of those questions ( ), you have to decide which is more important to you.