When the Canon 5D MKIII arrived at my local camera shop, I bought one and two good L lenses. It did not perform well for me. Too few keepers and the color, contrast and ISO performance did not seem right to me. After several days of agonizing, I returned to the camera shop and we took out a D800 and D600 Nikon, put the same lens (matched focal length and f) and made a number of test images both indoor and outside. Even the die-hard Canon sales folks said I should return the Canon, which I did for a full refund. Later, I saw the DXO score for the Canon and understood why I was seeing the difference.
If you have a relationship with a pro camera store, why not ask if you can take your memory card and test the three cameras at the store, go home and study the results. For me it made the decision easy to go with the Nikon.