I switched in January to Canon after being a long time Nikon user. The reason was I felt that the D800 was a lemon with the green screen problems and focusing issues. Even after receiving my D800 back from Nikon, it still focused poorly when compared to my previous D700.
The 5D Mark III has been a joy to use. The out of camera JPEGs, in camera vignette correction, dynamic range optimization, autofocus speed and accuracy, and high fps are better than any Nikon I’ve ever used.
The optics of the Canon L lenses are at least as good as their Nikon counterparts — dare I say better. Here’s the “but,” I like the build/feel of the Nikon lenses better. The biggest advantage Nikon has over Canon is the lens hoods and lens caps. What? Lens hoods and caps! Yes, lens hoods. Canon’s lens hoods don’t slide around the lens and click into place nicely like Nikon’s do. They are stiff, don’t go on easily, and don’t have a nice locking into place feel. Also the lens caps from even a year ago are not as nice to pinch and off. The newest lens caps copy Nikon’s. It just seems like Canon should have paid more attention to detail.
Also, the Nikon lenses feel more solid (made of metal) than Canon’s lenses. For example, the 70–200 mark II f/2.8 IS lens from canon optically is amazing and their IS works wonderfully. However, the lens shell feels like plastic. Whereas, the 70–200 vr2 Nikon lens feels metallic, solid, and inspires confidence.
In the end, optics, performance, and durability are what really matters. The optics of the Canon are wonderful, the performance is fantastic, and from what I have read and seen from professional photographers and sports shooters, the Canon are as durable if not more so than the competition.
In the end, for me the answer is clear. Go for Canon.