I’ve done my best to make this a fair comparison, but any test inevitably not only shows differences in lens performance, but also differences in camera performance and sensor size / resolution. The Canon tests were shot with an EOS 20D - 8 mp - the Nikon frames with a D70 - 6mp. The difference in sensor size means the subject size varies slightly. Nikon also quote a 1.5 crop factor relative to 35mm Canon claim 1.6x. The test crops are real world results - what you would get from each package taking the same picture from the same spot.
For more results on each lens use the links at the bottom of the page.