I rented a 5D Mark III and Sigma 35mm f/1.4 for this weekend's assignments from LensRentals.com and was very impressed by the results. The added benefit of renting the Canon gear was that I was able to use our department's 300mm f/2.8 lenses to shoot the football game against Georgia and soccer against Arkansas.
Compared to the D800, I felt like the 5D Mark III was much more responsive to use. I really enjoyed having 6 frames per second rather than just 4. Reviewing images was significantly easier due to the quick review using the wheel on the back of the camera. Changing autofocus points takes slightly longer than the D800 and the fact that you have to press a button before changing points can be a little bit annoying. Due to only having the camera for three days, I was not able to go completely through the menus to see if you can change this so I could be wrong.
EASE OF USE
Learning the system was easy enough. Many of the controls are where I am used to on the Nikon and many of the functions are the same. The ISO adjustment button is in an awkward place coming from nikon but it became easier and easier to use as I shot more. The multitude of different autofocus adjustments was a little bit confusing but made more and more sense as I used it.
Having a silent mode that is actually quiet was a huge advantage while shooting with the 5D Mark III. I was shooting portraits of radio show hosts while they were on the air and the silent mode allowed me to shoot during that time when I am sure that the D800 would have been far too loud. Also, during standard situations, having your subject not being able to hear the shutter is an extremely attractive feature.
High ISO performance was about the same as the D800. I was entirely comfortable going to ISO 6400 and even above in certain situations. Shooting the Canon and Nikon side by side at Lights on the Lawn , I may have preferred the Canon had I been able to use something other than the 35mm. In fact, many of the best photos that I took at the concert were taken with the 5D Mark III even without the versatility of having other lenses.
I did not experience any of the issues that I have had with the D800 in rendering reds, especially in concert situations. The 5D Mark III seemed to struggle a little bit with pink but other than that, I had no other issues with color.
All in all, the 5D Mark III is an extremely attractive option, especially with what I shoot 95% of the time. The extra FPS and responsiveness set it apart from the D800. While the extra 14 megapixels of the D800 are certainly helpful, the 5D Mark III outperforms in numerous other areas. It certainly makes the case for switching manufactures more and more appealing.