17mm Comparison: f11

Canon 16-35mm f11 (centre)	Nikon 17-35mm f11 (centre)

By f11 of course, the playing field has levelled considerably - especially on the 5D. A 1Ds II could probably tell them apart (see the conclusion for details), but centre frame they're level pegging.

Canon 16-35mm f11 (corner)	Nikon 17-35mm f11 (corner)

. . . and even in the corner there's little to choose between them at this aperture - though even here the Nikon's inherently higher contrast gives it the edge once again.