Having owned high end Nikkor and Canon L lenses, I will say I like them both equally for different reasons.. I find the canons are almost always faster to focus.. (excepting the 70-200 F/2.8 ED IF Nikkor which outperforms the 70-200 F/2.8L IS) I find the nikkors give a better quality image for the most part (excepting the 300mm F/2.8, where I find the Canon variant to be absolutely fantastic in every respect)
Horses for courses, different strokes for different folks.. The two "most common" brands for consumer/pro-sumer digital photography are pretty much on par...
Then you look at the likes of Leica, Comtax/Zeiss and Hassleblad where the quality level seems to rise somewhat... Hassleblad photographic equipment in my opinion is second to none... I've had the opportuninty to use one for a day, and it was the most amazing piece of equipment I think I've ever held.. (the price tag is almost as amazing as the gear!
)
For 99.95% of uses, Canon or Nikon will suffice though