I've owned both these lenses and I think they're both good. If the FL range of one is clearly better for you than the other, then get that one. But I think IQ is clearly (if only slightly) better on the 24-105. Not necessarily sharper, but "smoother" and more contrasty. Also having f/4 at the long end is a definite plus. Build-quality wise, the 15-85 is a lot better than the bottom tier of Canon lenses, but nowhere close to the 24-105. Mine had a bit of zoom creep, with a couple of "bumps" where it wouldn't stay in place unless you held it there. CA was only a minor problem (shooting raw with Lightroom). Vignetting was an annoyance, easily correctable, but I had to be careful with filters - no stacking. With the 24-105 (on crop frame) all this was a non-issue. 24-105 on crop-frame was a great walkaround FL range for me. I shoot Nikon now (and I'd have to say that the Nikon 16-85 is the lens the Canon 15-85 ought to be) but if I could afford full-frame, I'd gladly go back to Canon and those sweet f/4L zooms.