It all depends what you shoot, if you want F/F for sport Nikon, if you want it for studio Canon
I'd be inclined to say that Canon, with their 1D Mark IV and better autofocus speeds, are better at sport and Nikon better at studio ,with the D3X and the Nikon Creative Lighting System. But that's just my opinion.
Huh????? Canon: marginally(debatable) faster one-shot AF acquisition, but erratic tracking abilities.
Nikon: better ability to follow action and make entire sequences in-focus. Canon's 1D Mark IV's 1.3x crop-sensor loses out at elevated ISO setting to the larger, full-frame Nikon sensors, making the Nikon 200-400 f/4 actually a VIABLE f/4 lens for sports action. Having three, or even four, additional usable High-ISO speeds on the Nikon D3s means that f/4 lenses like the small,light 300 f/4 AF-S are much more useful than in prior years. The Sports-Illustrated cover shot of Drew Brees holding up the Lombardi trophy was shot with a D3 and the 200-400 f/4 VR-G Nikkor zoom lens...because the elevated ISO settings are so,so,so much better than what "the other guys" have now. AF is decent these days on numerous cameras with a skilled operator behind the eyepiece--I want better elevated ISO capabilities for news,sports, and nature shooting.
I see the Canon 5D-series as being a good low-budget studio option; the Mark II has an almost 22 MP sensor, for only around $2400 or so, depending on availability and the phase of the moon. But who knows when the "Nikon D800" or the next Canon model will be announced, let alone shipping, and let alone when they will be actually AVAILABLE for purchase in real, viable quantities. The entire photo industry supply chain has been constricting over the last few years, and the tsunami has made things even worse. Discussing a new Nikon or Canon body at this time is putting the cart in front of the horse.
(So, who do you like in the Super Bowl in 2014??? Pittsburg? New Orleans?)