I guess the 17-85IS does everything the 18-55IS does and then a little more so the 17-85 seems like a safe bet if the extra money isn't a problem.
Apparently, the 18-55 does what it does better than the 17-85 for the same range. It is however a budget lens, and doesn't feel that much different from its predecessor (the images look much better though). It is also perfectly paired to go with the 55-250 IS... I suppose it comes down to if you have to pay extra for the 17-85, or its the same price. Buying the lenses new, you'd get both the 18-55 and the 55-250 for cheaper than the 17-85.