EF-S 17-55 f2.8 IS
EF-S 15-85 IS (dont confuse this with the not so good 17-85)
the first gives much wider aperture availability and constant f2.8 which is very nice
the later is variable aperture and much slower but gives a wider wide end and much greater zoom range
they are similar prices
depending on which strengths you prefer either will be excellent
if you shoot alot of low light i would go with the first option if not so much and the greater zoom range is appealing go with the second
Agreed. Both will give you significantly better quality. If you are in the U.S. the EF-S 15-85 is much cheaper. It is on my 7D about 80% of the time. If traveling, I take only this lens and my 70-300 "L" and can cover just about anything.
Others may disagree, but in my opinion, in the wide range, there aren't practical EF alternatives. (Both of these lenses are good enough that if you later move to full-frame they should have decent resell value. )
+1, I also use the 15-85 and 70-300L
Both 17-55 and 15-85 a great lenses for a crop camera, L lenses in this range won't give you better IQ on your camera.
Worry about FF later - who knows what FF lenses will be available by the time you upgrade?
I know people who are happy with the 24-105mm f/4L on a crop camera, but most of them chose to complement it with an EF-S wide angle lens, because 24 is not really wide on a crop camera. With 17 or even 15mm, I don't miss the ultra-wide EF-S lenses (yet)