28mm Comparison: Distortion
As you might expect, both lenses increase in resolution as they increase in focal length, bending light more gently towards its destination. However, both similarly suffer from an increasing tendency toward pincushion distortion at the long end of their range, in common with most SLR zoom designs. Here then is how they both look at 28mm: (roll over the Canon image to reveal the Nikon frame) . . .
Once more the Canon is wider than indicated (or is the Nikon longer?), and both suffer from pincushion. The Canon 16-35mm has less distortion overall, but it's distortion is kinky - harder to fix. The Nikon's is more uniform in outline, but more severe in scale. Neither are close to a decent prime like the Nikon 28mm f2.8 AIS. And neither can be corrected without damaging the end result.
The correction process for barrel distortion downsamples the central area of the image; it throws away information. To inflate the centre of a pincushion-distorted image requires interpolation: making stuff up. The net effect is to destroy any fine detail in the centre of the image. Symmetrical barrel distortion is not a problem. Waveform distortion can be troublesome, but pincushion is a disaster.