So i´m in the market for a full frame camera right now. I only have FF lenses for EF mount, so switching would at least mean the loss of a lens or two (money reasons).
Right now canon is far behind others. Or lets just say it: Nikon.
The d800 is 2300€ the 5d3 is 2900€ and the Nikon just is the superior camera. If you downsize the image even high iso is awesome, af is good etc. It´s just a really good body and much more in the price range of a 6d.
I want to stick with canon but if they are going to make their first competitive body in like 5 years i really have to consider switching....
The d800 is in no way shape or form the clear superior camera. I compared every aspect of each camera while consulting for a well known photography equipment testing website:
5D3 has 0.5 stops more ISO which is a huge difference, half of that is from being under rated ISO wise (meaning when both are shooting at 800 ISO the 5D3 is ACTUALLY shooting at 1000 effective ISO) and the other half is due to better response to noise reduction algorithms.
5D3 has 50% more FPS (though only without the grip)
5D3 has better autofocus (though this will be hotly debated, and they are close)
5D3 has SRAW
D800 has crop mode.
D800 has better dynamic range, which evens out at normal ISO, and the 5D3 actually has better dynamic range at high ISO
D800 has more accurate color (although it's worth nothing Canon has more traditionally "pleasing" color out of the box as a result of it's inaccurate color, before editing)
Both have 100% identical indistinguishable resolution. What defines the resolution, quite frankly, is the optics more than the sensor. This has been proven by numerous sources, zoom lenses tend to be limited to just over 21 megapixels across 90% of their imaging area off the center axis, and there is no meaningful difference between a zoom lens image captured with the D800 or 5 Mark III in resolution, even when upscaling 5D Mark III images to 36 megapixels and comparing with the same zoom lens.
Prime lenses will sometimes but infrequently show a difference in resolution, but only the best prime lenses in the world (EXcluding the vast majority of Zeiss lenses even due to lacking resolution). And ONLY images with primes between f/4.0 and f/8.0 will show any difference in resolution, and then only at lower ISO (you can read more about how ISO affects resolution negatively elsewhere, or simply look at the lensrentals tests)
So in effect there is only a marginal difference in system resolution (and it's nowhere near 36 megapixels vs 22.3, more like around 22.3 vs 27 max)
My personal conclusion is simply put that the Nikon D800 slightly edges out the Canon in a studio or landscape setting (and Nikon's lenses back this up) and Canon's 5D Mark III slightly edges out the Nikon in documentary usage. (and Canon's lenses back this up).
Overall I've found both cameras to be more similar than they are different, and there is little compelling reason to go with either unless you really need something oddly specific. Nikon's lenses and grips are expensive so the price for a typical full kit is practically identical.
Here are a few very compelling lenses to go with either manufacturer:
D800 + 14-24mm 2.8 + 35mm 1.4 AF-S + 50mm 1.4 AF-S + 105mm & 135mm DC lenses + 28-300mm FX compact superzoom
(product, studio and full frame compact superzoom)
5D Mark III + 35mm f/2.0 IS + 50mm 1.2 + 85mm 1.2 + 24-70mm 2.8 Mk II + 70-200mm f/2.8 Mk II IS + 24-105mm f/4.0 IS + 200mm f/2.0 IS + 200-400mm f/4.0 IS TC + 24-70mm f/4.0 IS
(reportage, wedding and portrait, and super low light photography)