Separated at birth?
Nikon's big cat is now out of the bag, again, having had its news leaked all over the internet for the last few weeks. The D4 is coming. In February too, a 'month' earlier than Canon's new tour de force the 1Dx.
In reality these cameras should be available within days of each other and most likely (more so for the Nikon who have been dealing with more troubles than Canon) in limited quantities for the next 4-6 months. Which may not be a bad thing, these beasts are expensive. $6000US for the Nikon, $6800 for the Canon.
So assuming you don't have a climate controlled storage area for all your (insert brand) pro lenses which one is going to tempt the average shooter more? Spec wise they are almost indistinguishable from each other. 18mp vs 16mp, 10vs 12 fps, both have masses of AF points, high pixel count metering/tracking systems. Both are weather sealed, built like tanks and weigh a ton (1.3KG or so). Canon has given their beast dual CF card slots (about time!). Nikon have jumped , rather early, onto the XQD bandwagon.
Do we need to ask which is better? Is there any point? There both absolute top class tools for serious pro togs, I'm sure there will be many a flame war around over these but if you cant decide, toss a coin! (then buy the Canon).