I don't want to get into an argument, puddleduck, but if you haven't used the Canon 400/5.6 then you really shouldn't make comparisons like this. Good though that Nikon combo is, the Canon really is *much* better. Lighter. Faster. Sharper.
The 400/5.6 niche is an interesting one though. There are *lots* of relatively affordable ways to get to 400mm at f/5.6, such as:
* Canon or Sigma 400 f/5.6
* Canon 100-400 f/4.5-5.6
* Nikon or Sigma 80-400 f/4.5-5.6
* Canon or Nikon 300 f/4 + 1.4xTC
But going longer and/or faster immediately starts to cost silly money. I guess that's because 400/5.6 fits within a 77mm front element, and there must be economies of scale in using components which are the same size as lots of other lenses. So anyway 400/5.6 is the affordable limit for a lot of photographers, and I'm surprised there isn't more competition there.