Just to add my 2 cents, and I'm no pro while at the same time no amateur- I've been shooting for about 5 years for a hobby and never once ventured out of manual mode (literally learned only manual and still have no idea about the Av, Tv, and P modes on my 20D)... but I am one of those freaks who insists on getting the best he can afford. So that being said, I had been throwing this same tossup of lenses over and over in my mind for the greater part of well, the last 4-5 years, reading dozens of reviews, viewing dozens of video reviews, looking at hundreds and hundreds of images produced by both on Flikr forums, and I finally made my choice back about a week ago when I pre-ordered my 5DM3.
I decided, at this point in my life, not being a pro, shooting family, my dogs, and for fun and NOT to make money (while at the same time doing a LOT of indoor shooting with really phenomenally low lighting in my house which I really should redo when we remodel, but I digress), that I would go with the 24-105. I mean, I was in the throes of a decision before the Mark III was even a blip on the radar screen and I was only aiming for the Mark II, even before and after the 7D was released. Having viewed and the knowledge of many higher ISO images of the Mark II and now the Mark III, I've decided it's not that vital for me to have the 2.8 speed over the f/4 because I can raise the ISO a little more and still get my shots with negligible increase in noise. Of course, this is coming from a guy who learned shooting in low-light with a 20D... where pretty much anything beyond 400 really bit the big one, in my opinion- at least in the darker rooms. Then again, I also have the 50 1.4 which is pretty amazing although I haven't really shot much with it, and I only got it as a replacement to a broken 50 1.8 which was like money with every shot. Still considering getting another 1.8- I mean, it was razor sharp.
I also plan to eventually buy the 24-70 in the long run when prices get more manageable but for now, yea, I can handle a better ISO camera than the Mark II and f/4. But I also recommend just throwing in the 50 1.8 for the hell of it cause, hell- it's 1.8, razor sharp and the bokeh is pretty decent if that's what you like- all for $100 it's a no brainer.
I stand by all that I said before, but a thing of note to you:
There are different lenses that the Mark III's new auto-focus system works well with, and lenses it works above well with. Basically, they have a list that states which lenses can take advantage of the obscene superior quality auto focus points, and basically their whole list of current lenses is listed as to which lenses fall under what class. A is the best, and it goes on from there to B, C, down to H.
If you look at the free to view online manual for the 5D M3, you can see the lists on what is page 79-84 of the printed version. The Group A is filled with mostly primes, some zooms including the 70-200 2.8's (version I and II). Group B which has the best central focus is much smaller with the 24-70 2.8 (version I listed, and I couldn't find mention in any list of version II, I suspect because it's not yet released..? or is it...) along with some primes, and Group C contains the 24-105. To specify what they say is best, Group A has a string of Dual cross-type Auto Focus Points, B has a singular, central one- and Group C only has Cross Type Auto Focus Points.
I don't know how this all relates to the actual quality of sharpness your camera can help the lens attain, because I'm coming from a 20D and haven't researched all the details of said Auto Focus points, but I just wanted to throw this in after having learned and messed around a little with my camera tonight, as I was unaware of the difference. Luckily, I have a 50 1.4 to test the Group A with and compare a 50mm length on the 24-105 Group C with later. Good luck in your decision!