I sent back two copies of the Sigma 35mm F/1.4 and kept the 24-70 F/2.8II because at F/2.8 it is sharper (at least my copy is) and it focusses a lot faster and more accurately.
I have to say, my 24-70 is the only lens I've had on my Canon 5D MK3 that I've liked and not sold (apart from the 70-200 F/2.8 II).
I've tried or owned a lot of L lenses (50 F/1,2, 24 F/1.4 II, 16-35 F/2.8II), Zeiss lenses and even the new Sigma's and Canon IS primes but nothing pleases me as much as the 24-70, and I'm more of a prime guy that a zoom guy.
Don't you miss that crazy 3D feeling the 24 gives @1.4? I've been going going back and forth between keeping or selling the 24 and swapping for the 24-70, but every time I try the 24 @ 2.8 to see what I would actually have with the 24-70 I don't like the look. I hardly ever use it any smaller than f2. and doesn't the 24 have much less distortion? But I guess it comes down to what you shoot..
I'm really keen on the 24-70 for shots with my Quadra light though, the difference in perspective (and smaller apertures anyway) makes it very high on my list...
btw, as I mentioned my 70-200 that was for service. They replaced two IS-units, the AF-unit, some ring internally in front, adjusted the glass, calibrated AF and two other parts. It still sucked at 200mm and not awesome for the rest, seems decentered also, so I had a guy try it and he loved it so I gave him the servicereport and a good price and bought myself a new one, will hopefully arrive tomorrow. Let's hope I just got a bad one and that the new one is as good as the very first copy I had a while back.