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I personally haven't used either, but from what I've read the 15-85 does not give up much optically or in terms of build to the 17-55 (actually I've heard that the 15-85 is better built; maybe someone else can confirm). Since you seem to use the long end of the 17-85, I'd think the 15-85 would be a good bet, plus you'll gain the extra 2mm on the wide end (which is pretty substantial).