I got flamed by some for this comment a few months ago,...but I'll say it again: I think Canon has a "better" lens lineup, especially for the advanced amateur, and particularly for the amateur action shooter, whether sports, birding, aviation, racing, etc..
The 100-400 IS USM is the poster-child for my "opinion", but there are other examples.
If I was starting from scratch building a new system, I would go through hell deciding. I LOVE Nikon bodies, and this preference over Canon is reinforced everytime I pick-up a Canon body. But, Canon's lens line-up is more tailored to my needs.