Nikon d810, nikon d7200, canon 5d mark 3 which 1 to buy?

Nikon d810, nikon d7200, canon 5d mark 3 which 1 to buy? - 1

Lets see.

You have listed two full frame digital SLR cameras and one cropped sensor camera

If you already have a Nikon digital SLR, then spending the huge amount of money necessary to buy a Canon 5D, Mark III and associated lenses just makes NO sense economically

In any case, here is a link that shows how the sensors of those three sensors compare when it comes to colour depth, dynamic range and low light photography (high ISO settings)

The 5D Mark III is rather old and is on it's way out. So, unless you've got a need for a specific lens that only Canon makes, then I'd go with the D810 over the 5D Mark III. For example, Canon's 70-200 f/2.8L is the best on the market. Nikon's version suffers from terrible focus breathing causing it actually be at about 140mm when focusing close to about 10-15, even though the lens is set to 200mm. Canon's mount is larger than Nikon, allowing Canon to design brighter lenses like the 50mm f/1.0 and the 85 f/1.2L. Because Nikon's mount in smaller, Nikon can't produce lenses with apertures larger than f/1.4. Also, because the Nikon lenses requires a larger gap between the rear lens element and the sensor than Canon does, you can use Nikon glass on Canon bodies, but not the other way around.

The D810 with it's 36MP sensor, is compromise. You gain resolution (the ability to print larger) but sacrifice high-ISO performance. However, that should be fine since the D810 is not designed for those who need great super-high ISO performance. It's designed for those who need high resolution such as landscape photographers who are almost always shooting at ISO 100.

The D7200 is not in the same class of cameras as the D810 or the 5D Mark III. If you don't mind sacrificing the ability to use ultra-wide lenses such as the Nikon 14-24mm to their fullest, then the D7200 might be a better choice. You do sacrifice which lenses you can use with DX bodies. You will not get the same level of sharpness out of an FX lens like a 70-200 f/2.8 when used with the Nikon DX bodies. There are very few pro-level DX lenses, so I personally would not go with a DX body from Nikon unless I was a wildlife photographer and really needed the additional focal length that the 1.5x crop factor provides.

Depending upon what you do or want to do, any one of these could be the better camera. Or, it could be that none of these are good for what you want to do.

Wait for the D500 to become available. Get it bundled with the 16-80mm f4EAF-S VR.