After I bought 5D III, I tested both lenses. The 16-35 II is sharper wide open, and the f/2.8 is nice for handheld low-light shooting. DoF isn't much of an issue with a UWA lens.
You will need to stop down f8 or so on the 17-40mm, otherwise the corners are pretty mushy when shooting at wide open.
At the end, I bought 16-35 II.
The 17-40 sharpness drops fast at the corners of a FF camera. With APS-C size sensors however the sharpness goes from edge to edge. And to clear up some confusion, a F/1.2 or 2.8 or f/4 lens ALL set to f/5.6 will be letting in the same amount of light. it is only when they are set to their wide openings (f/1.2 etc...) that they are letting in more light. If you want a sharp landscape, usually around f/6.3-f/8 are sharpest, and in this case the 16-35 wont be letting in any more light than the 17-40. Hope that helps!