Any lens that has extending barrels will get dust. Having a dust-free lens only lasts a few outings
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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.