When it comes down to it, some Nikon glass is better than the equivalent piece in the Canon lineup, and vice versa. The advantage of Canon over Nikon is in the auto-focus, moreso with the big long lenses because in the EOS system, the focusing motor is built into the lens, not in the body, so you get the right size motor for the lens. Nikon has more recently compensated for this with their big lenses by adding booster motors to speed autofocusing. This also leads to reduced battery life by having two motors to run, but this is a problem unlikely to be observed by most of us - if it rears its head at all, it only happens for the pro action photographers who are firing off hundreds if not thousands of shots in the course of a single sporting event, all of which are shot with 400 f2.8 or 500 f4 lenses.