Hi, I'm new around this forum, and I think I should start by sharing some thoughts that I had about this.
I think we should all really look at the issues that plague the Nikon D800 right now. I'm pretty sure a lot of people are swooned by the ridiculously high megapixel count (which I honestly think most people would not need, because it makes processing take longer, and takes up more space!) but you must realise that a lot of people are facing problems with the D800 right now. There is a growing list of issues ranging from issues such as the camera software locking up when reinserting batteries, to even oil droplets/debris splattering onto the sensor after a few shots (rather reminiscent of Canon's 1D III issues in the past that caused it to be quite a failure, actually).
Just look up the nikonrumors forums and dpreview forums. Unfortunately, I'm new, so I can't post links just yet. *sigh*
Search for 'D4/D800 issues' on google and the top result should direct you to the thread on nikonrumours forum. All that stuff is there.
Now, comparatively, the 5D Mark III's screen light leak (non-)issue is really minor (honestly, who shoots with the body cap on?) compared to major serious functional issues of the D800 right now.
I'm not saying that D800 is a bad camera, I'm just highlighting the issues that we all have.
Let's now assume that the D800 doesn't have these issues in the first place, let's compare the "profiles" of the 2 cameras:
5D Mark III has better noise performance than the D800 when using higher ISOs. When photographers shoot events, especially wedding days, the 5D3 is the better choice of camera because a lot of events require higher ISOs to be able to shoot in many of the low light settings. Dynamic range wise, remember that 5D3 surpasses the D800 once you hit ISO 800 and above.
While higher megapixels mean greater resolution of detail and the possibility of down-sampling, think about the consequences of your workflow. While the higher megapixels of the D800 will give certain advantages to the photographer, think about the size of each photo that you take. If you're shooting RAW like what most professionals would do, you'd be dealing with MASSIVE file sizes! I mean, not only do you waste space, but you also waste time transferring photos over from your memory card. Of course you can opt to save photos of smaller sizes, but then you're throwing away the advantages that such high megapixel counts give you, then you're better off with the 5D3.
It's only a matter of time. After a while, Canon or Nikon will definitely respond to each other's releases. It may take a year or two, but the comparison between the 2 companies is really neck and neck, not leaps and bounds.
Consider your losses by switching over. If you already have invested significant amounts into L glasses, you'd be making a loss by selling them and buying new glass from Nikon! Most professionals also don't do that anyway.
Both cameras actually cater to different needs. The D800 is a better landscape camera that is meant to be used on a tripod, and not really event coverage, because it has better resolution of detail, and better image quality only at the lower ISOs. The 5D3 on the other hand, is said to be the best photojournalist's tool yet. Not as bulky as the 1DX, yet has the same 61-point autofocus as the 1DX, and has superb high ISO noise performance, delivering a more consistent quality of shots throughout the ranges of ISOs.
If talking about needs, I think 5D3 would serve most people's needs better than the D800.
Hope this helps!
