Wow. A lot of the responses have utterly missed that these two zooms are rather different in their performance, even at small apertures. I own the 24-35 (along with primes within it's range) and I used to own the 18-35, so I have a pretty good idea of the lenses characteristics.
Outside of the obvious focal length coverage differences it comes down to these two points:
1) Are distant corners and edges important to you in what you do and are you willing to give up some of #2 below to get it?
2) Is central zone rendering (subjective) and micro-contrast at close to moderate distance subjects important to you (and you are willing to give up some of #1 above to get it)?
If #1 is your thing, the 18-35G is clearly your lens. If #2 is your thing, the Sigma 24-35 is your lens. I find them actually substantially different.
I don't know (specifically) what you shoot, so I can only tell you why I have the 24-35 around at the moment. I shoot studio (mostly), often with wider lengths (35-50) and landscape (secondardily) at the 20 through 135mm range most of the time, with a lot of time spent in the 20-35mm end of things. I am very picky, and I shoot a D800E. When I'm out shooting landscape, I bring a prime collection along - 20/1.8G, 24/1.4 Sigma art or Nikon 24/1.8G (I have both in house at the moment), and 35/1.4 Sigma art. For landscape, the only reason I would bring the 24-35 Sigma along is if I think I'm going to be in a very specific situation where my main subject is predominantly in the center and I have a lot of things in the near corners that are important. I generally won't carry it because at distance, even stopped down, the edges and corners of distant objects are *not* satisfactory for me. The primes are better. The 18-35 I used to have is better. The 14-24 is better. Etc. The 24-35 has a distinct field curvature that favors things near at the definite expense of things far, and thus, if your scene fits that, you'll be in heaven, but on the other hand, if your scene doesn't fit that, you won't be happy - at all. The Sigma 35/1.4 Art prime is better at distant corners and edges, as is the 24/1.4 Art or Nikon 24/1.8G.
In the studio I like the 24-35, and use it mostly there, because the central region is superb - it has a *different* subjective rendering (not better, not worse - just different, as in, choices are good) than the Sigma 35/1.4 Art, which is my most used lens in the studio for what I shoot. Very broadly speaking the Sigma art prime will trend very slightly towards lean and detailed (subjectively) and the Sigma art zoom will trend very slightly towards romantic and slightly bold(subjectively). The prime is slightly sharper in the center, in the studio, but the zoom is right behind it. For studio work, I am not concerned too much about edges/corners and in fact near-corner favoring field curvature is often beneficial.
The 18-35 is more "even" across the frame at distance, and thus it's better for distant corners/edges. However, it does not have the (subjective) "life" nor superb central zone sharpness of the 24-35/2 zoom. It will show some minor faults on a 36mp body in the corners and edges, but not as much as the 24-35/2 zoom, and on a 24mp body, it's likely an excellent fit (although I'd still go with the prime kit mentioned below)
So it's a matter of which is more important. The Sigma art zoom is also very large - 82mm filters, and much heavier/larger than the 18-35. Honestly, if I were trying to travel light and maintain very high quality, I'd carry the 20/1.8G and 24/1.8G Nikkors (both are bloody excellent lenses, particularly that 24 which is IMO the best of the entire 1.8G lineup and easily so) and possibly a 35mm prime of some flavor - the Sigma 35/1.4 Art is heavier, but the Nikon 35/1.8G FX and also the Tamron 35/1.8 VC are also extremely good choices. I would run with that three prime kit over either the 18-35 or 24-35 zoom for landscape every day.
-m
