Nikon and canon lens, are differnet, not by the quality control, I supose is the same, I use Canon lens L level only, and I very happy with the result.
Compare the N to C lens, is the same to compare Leitz Vs Zeiss, the Leitz like more the canon more smooth, and zeiss like the nikon more hard, but any one are the same qualtity.
The decision, in may case, for canon EOS is the full 24*36 capabilities in the 5D and the 1DS, more valuable that the possible quality difference in the quality contol of the lens.
Cheers
Cesc