Switched to the D800 coming from the 5D Mark II. IQ-wise, there is no comparison to the D800, I would say, but I'm saying that only from what I've seen in tests.
The high-ISO image quality is better with the 5D III in *practical* shooting, but better with the D800 when it boils down to theory. This is because the D800's amps and noise reduction blabla are tuned to the red channel, vs. blue/green on the 5D III. There was a test somewhere where this was shown, I have to see if I can find it again.
The big difference between Nikon/Canon is the handling. One thing is it's all literally "the other way around" with Nikon: The lens bajonet direction, the dot marking on the lens, the exposure scale in camera. It takes a while to adapt :-)
Unless I had a lot of time and some spare money that I didn't know what to do with it, I would, in your situation not consider switching. Why are you? You have the latest gear and it's considered top notch. There would only be two reasons that would make sense:
- You really need that focal legth package of a 14-24, as in: in one lens. The IQ is great, but not as legendary as you can get the impression when you hear all that raving. Read some (real) reviews and look at distortion and sharpness values compared to other lenses. You can get great alternatives to it if you're not specifically looking for this exact combination as a zoom-lens package.
- You want the resolution of the D800. That's why I switched, because I wanted to be able to crop and still have high-res pictures left. The IQ itself is also marvellous, but then again, I doubt that this difference would really matter when you sell your pictures to someone. Plus, your lenses are (supposedly) excellent IQ, I doubt you'd see any improvement with the same gear with a Nikon badge on it.
Why not to switch:
- The D800 is missing the C1-3 comfort modes of the Canon. If you're using those heavily, don't switch. The memory bank architecture is a complete fail in the D800, non-usable. Then again, it has to offer other nice features handling-wise, but it's all a matter of taste.
- Last thing: The quiet mode on the D800 does nothing but separate mirror up and down noise. It's still loud. And the D800 is as loud as a regular SLR, in fact, louder than smaller Nikons. The quiet mode on the 5D III is actually a miracle compared to that. If you need quiet, stick with what you've got.
Summary: Only switch if you want 14-24 in a zoom lens or want the high resolution.
Do not switch if you rely on C modes in your Canon or the quiet mode.
Hope this helps.