Do you know which is better to use? Let me answer the questions above.
Look and Control
Canon 5D Mark III weighs 1.9 pounds is little lighter than the Nikon D800 which has 2.1 pounds weight, That’s not a serious deal as they have almost same body size.
The control has no big change at Nikon D800, The rotating dial on the left maybe the only one very obviously changes. That means the Nikon hopes last the traditional easy control. Compared with D800, The 5D Mark III upgrades a lot from 5D Mark II, You can find the new 100% viewfinder and the back button that is A bit like Canon 7D. If you are an old user of EOS series, You need time to grow into the control.
How about the performance?
There is no question that the performance of Nikon D800 is the best between the 35mm cameras, And if you like shooting movies, The mini HDMI Port can be very powerful, it also offers 4:2:2 hdmi signal that other cameras are hard to achieve. With the 36.3-megapixel FX-format CMOS sensor and EXPEED 3 image Processor, Although the color noises are not as low as we expect but very well for the price. The weakness of this camera is the high demand on lens and tripod.
The Canon 5D Mark III has 61 points focus area that is improved the focus speed and accurate a lot, And the continuous shooting speed of this camera is 6fps which is beyond any other EOS cameras except Canon EOS-1D X 18.1MP Full Frame CMOS Digital SLR Camera. Although the 22.3 Megapixel full-frame CMOS sensor is not greater than Nikon D800, But the hot pixels are less at High ISOs. Just like D800, You should pick a better lens to fit this camera.
Nikon had been beaten down by the high pixels strategy of Canon, Though the Nikon D700 has the faster continuous speed and better color noise reduction, But the winner always was the Canon EOS 5D Mark II.Finally the Nikon produced the D800 which is the pixels winner at APS-C DSLRs, However it has the weaker color noise reduction compared with Canon 5D Mark III, If you want to get the detailed images, and find more about the two camera, Read the review about the two product below.