Hi, I own the 24-105 and like it a lot. I do have other lenses where I avoid the largest aperture, but that's not an issue with this lens. In high contrast scenes it can show quite some chromatic aberration, and there’s also significant barrel distortion at the wide end. However, the RAW-conversion software corrects for these flaws basically without effort.

If you look really close, the 24-105 is a little less sharp at the long end (like many zooms), but still quite sharp. Basically you don’t have to bother about sharpness with this lens - it’s sharp at all apertures and focal lengths. The flaws I see are the ones already mentioned.

If you are interested in "sharpness as a function of aperture and focal length", I really recommend the test graphs at slrgear.com - they describe the details better than I can do.

I never tried the 24-70, but I have a friend who owned it and was equally happy with it as I am with my 24-105. I’ve got the impression that they both have really good IQ and that IQ is not the differentiator between them. Instead it’s the obvious difference in their specs: The 24-70 opens up to f/2.8 while the 24-105 only goes to f/4 but has some more reach and excellent IS. What you prefer is up to you.