Having had a solid two month's experience with D800, I feel it's safe to say that 5D II/III shooters who poo-poo the D800's wider DR are speaking from inexperience. They just literally don't know any better.
That said, I sold my D800 in favor of the 5D III for several reasons. Here are a few:
1. For wide-angle landscape and architectural photography I rely on Live View for focusing. The D800 has what I regard as a seriously poor Live View system that skirts the line between usable and unusable: quite usable in good light; unusable in low-light with narrow lens apertures. The quality gap between the D800 and 5D III LV systems is immense imo.
2. I love the Canon wide-angle tilt-shift lenses (perfect for architecture and landscapes) and Nikon is not currently competitive with wide-angle TS lenses. Also the 100-400mm is a great bang-for-the-buck all-purpose wildlife lens that Nikon has no similar quality : price answer for.
3. While the D800 sports an advanced AF system, the 5D III is surpassingly good. All-in-all, the 5D III bests the D800 on ergonomics, in my view. And, ultimately, image quality is connected to ergonomics. It doesn't really matter if you've got 36 MP and and huge DR camera if you can't get your subject in perfect focus because it's dark and the LV system isn't usable. So, in a round about sort of way, the 5D III can even out the image quality score because it makes it easier on the user to capture the image. And because the 5D III has better ergonomics I think it's just more fun to shoot than the D800. But, I'm not suggesting that the D800 has bad ergonomics, just that the 5D III is ergonomically superb.
It's hard to quantify, but don't make the mistake of dismissing the connection between ergonomics and image quality. I watched an excellent Youtube presentation comparing the 5D II and 5D III for shooting wildlife. The user experienced a 25% success rate capturing birds in flight with the 5D II, but had something like an 80% success rate with the 5D III. That's a huge jump in overall image quality in a way that doesn't get discussed much.
I very much enjoyed my time with the D800 and truly wish I could afford to shoot both systems (by afford I mean both my time and money). For now I've decided to shoot only Canon and to do so with a 5D III for the above-mentioned reasons. At first I was skeptical that dpreview "conveniently" scored the two cameras with the same final score; especially since the D800 is the clear winner on image quality. But, now having used them both, I would rate them the same way. They are both awesome cameras with unique strengths and weakenesses.
well TS is shooting with a crop camera, so not sure how much dynamic range he really needs. My guess is that he saw the FM or youtube comparisons. The biggest factor is what he shoots and is he willing to make compromises. The D800 fits my style better as well, but the D600 a much closer match if it ever materializes.
I'm done chasing gear at this point, at a happy medium