Opinions on Nikon D800 and Canon 5D Mark III?

Over the past week I've been reading bit by bit on which one, for the foreseeable future, would be a better purchase. Such objective, though, is quite unfair, as I'm well aware what both have strengths that are not necessarily parallel with each other.

I've had a 7D for over 3 years, and I feel too limited with its cropped sensor, therefore, I have saved up to make the leap into full-frame. I've read very heated arguments regarding these two cameras, each one having a pro that is hard to give up, while also having a con that is not easily dismissed.

Without going into too much detail, the consensus seems to be that the D800 is amazing at stills, and barring any low light scenarios, is also quite sharp and pretty for video. The mark III on the other hand seems to be baby steps away from the mark II, and for me, its most attractive feature, other than my familiarity with Canon, is the extremely attractive low-light performance.

My purpose for this device will be mixed, I will take stills, and I will shoot video, both largely in equal amounts. However, I still don't know which one to buy. I'd like to hear some opinions if you own either of these cameras, or if you own a different one and would like to share the reasons as to why it is as good as it is.

I picked the D800 because I already have a full Nikon system

Unless you have mostly Canon EF lenses, moving from the 7D will require you replace any EF-S lenses since they can't be used with any of the Canon full frame cameras like the 5D, 6D or 1D series cameras.

Fortunately, I can use any of the Nikon DX lenses on my full frame cameras. Those cameras automatically crop to the ASP-C image size when using those lenses

You need to spend some time on the Nikon and Canon websites as well as DP Review were you can read professional reviews that take an in depth look at the features of both camera

Spend some time before you make any decision, since it will influence your future purchases for the next decade.

It may help you to visit a proper camera shop and hold both cameras in your hands

NOTE: Most advance amateurs and pros, have both a full frame and cropped sensor camera. The extra reach the cropped sensor cameras provide when using telephoto lenses makes APS-C sensored cameras a valuable tool.

There's no doubt the D800 is a superb piece of equipment but I have too much invested in Canon lenses to jump ship and a real gut feeling that Canon will match or surpass the D800 sometime soon. In the interim I will make do with my 5DM2 + crop and invest in more glass.

Nikon D800 is great if you really need that amount of resolution, but that's about it. Too many pixels and it ends up performing like a full frame version of a d7000 at high ISO.

You are correct that the mark III isn't a huge improvement over the mark II when it comes to the image sensor, the AF is a lot better on the mark III. It's too much money for the image sensor performance. It sells in big numbers because of the huge number of Canon users that have so much invested in Canon glass.

If you made the Mark III and changed the mount to something else like Sony Alpha or Sigma, nobody would buy it, 3, 500 US dollars at debut is too much for an image sensor that isn't any better at high ISO than a Pentax K5II with a Sony Exmor sensor that kills it when it comes to dynamic range, and costs one third of the mark III. But it says Canon on it, so it sells.

If you have a significant investment in Canon, and you can live with a bare bones full frame camera, the 6D is the way to go. You will save over a thousand US dollars. On the downside you have no weather sealing really, one single cross type AF sensor, single memory card slot.

But the image sensor is better than the mark II and III. How Canon screwed their customers this bad, I have no idea. They also boned all the crop sensor customers by putting the same mediocre sensor in the cheap Rebel, the 60D and your 7D. When you pay more, they don't give you a better sensor.

If you stay Canon, just get a 6D. If you don't need HD video and some features found on new dSLRs, nothing beats a used full frame Nikon D700. That sensor still crushes new, more expensive dSLRs that are 5 years newer, proof that a good sensor with huge pixels still dominate. If you want the best combo of high ISO performance and HD video, the D600 from Nikon is a better choice over any Canon full frame with the 1D X being the 6, 000 dollar exception.

The glorious 36MP of the D800 will not be useful unless you are willing to invest in the lenses that can sharply focus well enough to have detail to be captured at that magnification.
If you want to be able to walk around with the camera without a cart full of lenses, I would consider that the Canon 24-105mm f4 is reputed to be sharper to the edges than the Nikon 24-85mm or the Nikon 24-120mm.