I look at it this way, what everyone is talking about at very small differences. Overall BOTH cameras are going to produce great images in the right hands and utter rubbish in the wrong hands. For me it boils down to this.
Even if the D800 is better it will be by a marginal technical amount that will more than likely not be noticable in images and definately not noticable in print.
With so much invested in canon glass (as with alot of other people on here) changing teams would be a silly haircut to take to change to Nikon.
Sure this round Nikon win the MP Bragging rights and some numerical theoretical DR bragging right. (How many people waste all this image quality and inbuilt DR by running 9 bracket exposures at 0.5 stop through a trash compactor like photomatix etc? Can people not see how the image quality is decimated by these programs FAR beyond the miniscule differences between a D800 sensor and a 5Dmk3 Sensor, DR is becoming a crutch. Ok overprocessing fans smite away again
This stuff while interesting to read about being technically minded does not mean that the differences between the cameras are going to be noticable or even measurable in real world work and deliverables.
Step back and look at the bigger picture, then go and take some enjoy the camera and the shoot, after all isnt that what most of us are here for anyway?