The 24-70 is used for the same type of shots as the 24-105, it just has a bigger aperture and therefore lets you shoot moving or active subjects in lower light.
As far as image quality goes, I've owned the 24-70 for about a year and used the 24-105 and they seem pretty identical with maybe a slight edge towards the 24-70, although I much prefer the 24-70 due to the f/2.8.
It's a great walk around lens, but heavy. I owned one during a vacation I took to Hawaii and sold the lens a while after that (and bought the 50mm f/1.4) but for my last trip to Hawaii I rented one for the three weeks to have, it's a really good lens for just all around photography when you dont want to be switching lenses in and out.