I've heard from some pros that the 24-70 EF is not that good of a zoom. Juza's test target photos, plus the field samples, show that the 24-70 has pretty bad corner performance. The photo here
http://www.juzaphoto.com/shared_files/articles/canon_24-70/canon_24-70_24mm.jpg shows absolutely horrible corner performance....just look at the lower left hand corner....it's dismally bad. That lens is not good enough for a 21MP, FF sensor camera.
I only own the 24-105 L ,and do not own the 24-70 L zoom, but Juza wrote that he had tested two samples of the 24-70,and both were bad; the first one was apparently so bad that he thought it was a bad sample, but now he has tested another 24-70 and as he wrote, the results are the same...and other people who are really "into" wide zooms say the 24-70-L needs an update...makes me think that it's just not optically that good. The wide-to-short telephoto zoom has never been "easy" to design or make--the lens has to cross types, from a wide angle to a normal to a short telephoto, and at wide angle focal lengths, depth of focus at the film plane (not to be confused with depth of field) can mean that incredibly small errors make for badly focused, soft images, and the wide angles of view (94 degrees IMMSMC at 24 on FF) make it hard to fully correct all optical flaws. And then,to top it all off, the lens is made at f/2.8 speed, which adds additional difficulties. The slower 24-105-L benefits from an f/4 maximum aperture, which is slower, and makes the design easier to build.
I dunno...I'm not enamored of the 24-70 or 28-70, heavy, tank-like f/2.8 zooms that weigh a kilogram or so, like the 24-70-L. I've owned a couple of these "coffee can lenses", and I just do not like the size and weight and the huge obnoxious profile they have, and the way they make the camera nose-dive all the time. What's weird is that Tamron's 28mm-75mm f/2.8, which is made of industrial plastics, is so,so,so good that it's more flare-resistant than a Canon 50/1.8, and it out-resolves many primes lenses, and is such a legendary lens that both Pentax and Sony have licensed it to be made for them,with their own proprietary style of rings and rubber coatings....and it offers about 90% of the optical performance of a Nikkor or Canon 24-70, but at 1/3 the weight and what is it now? 1/5th the price??