It is a very good question indeed. As a very technical shooter, who likes to twiddle with all the knobs buttons and settings on a camera, and who after all is teaching that the camera does not make the photographer, some of my decision was based on practicing what I preach.
I recently upgraded from a 5D mark II and a 7D, I have always used two bodies, as each had its advantages and disadvantages, but they complemented each other. The 5D II was great in low light (for its age) and the 7D was a focus monster for the money it cost at the time. I originally bought the 7D to replace an aging 1D Mark II.
In 2013 just after the 5D mark III was released, a friend of mine who has a great mix of bodies and lenses (1DX’s and 5D III’s) asked me to help him out as he was double booked. It was motorsport and of course I said yes. As payment he offered me his 5D III for about a month to have a bit of fun with. I ended up taking about 5 to 6 thousand images on the camera as well as doing a wedding. It was fantastic it focused like a machine (better than the 7D and had amazing capabilities even at ISO 25600! It was sorted when I could I was going to replace my 5D II with one. So why did I buy a Canon EOS 6D?
When Canon released the 6D, it was full frame, it had the same ISO capabilities as the 5D III, which was one of the things I really “needed” it didn’t have the focus system of the 5D III but the important thing was it was much better in low light focusing than the 5D II, the biggest let down of the 5D II (in my opinion). I had seen a few friends buy them, and I asked them how they liked it, most were all amazed by its abilities.
This advantage seemed so trivial at the time, but I have found it to be a very great little feature. WIFI! Who would have thought, I can connect the camera to my phone, and have a shot on my phone and be able to email it to a client (in low resolution) within seconds of it being taken. Yeah it sounds trivial but it’s amazing.
One disadvantage was the need for new media cards, as I had been using compact flash, and now needed SD, they are larger in capacity and faster in read / write speed for the same sort of money, so that was that much of a disadvantage.
The most obvious thing of course was cost. The 6D was conservatively half the price of the 5D III which is a lot of money to some people and can be hard to justify. The bigger question was “is the 6D half the camera that the 5D III is?”
My simple answer is NO.
My longer answer is this, for the type of work that I expect the 6D to do (I do not need the amazing focus abilities, I bought a 7D II for that) I do not need the 6D to take more than 4 frames per second (I bought a 7D II for that) I need the 6D to focus in low light where I would be using the camera with no flash and very high ISO. And this it does amazingly.
Here are some images taken on my 6D some in my testing phase, this is where I push the camera to see what it is capable of.
I will explain what aspect of the camera I was testing with each example.
ISO 1600 F2.8 1/15th and no tripod. I wanted to test the abilities of this camera with high ISO and a mixture of bright and dark areas in the same picture. Usually with high ISO you see the grain in the darker underexposed areas.
ISO12800 F2.8 1/250th, I really needed to be able to freeze the action here, and to do that I needed to use a faster shutter speed, also the athlete was moving, and was back lit, so I was testing the focus tracking in these poor lighting conditions.
ISO 12800 F2.8 1/180th trying to catch the cat in front of the camera I needed to use a fast shutter speed, to do this indoors at night, I needed a very high ISO, this image has very little noise for a shot at ISO 12800.
ISO 1600 F1.8 1/30th taken with a 28mm F1.8 lens (yes that is not an L lens) I wanted to see if the focus would work on fast moving objects in very poor light while panning. Cars were doing about 60 km/h.
ISO800 F1.8 1/60th taken with a 85mm F1.8 lens (yes that is not an L lens) again I was testing the highlights against the darkness in the spots. And focusing on a moving target that is backlit.
Footnote: the 6D has a custom function menu just for its focus system (the 5D II had one of these that allowed you to turn on 6 extra focus points to assist the centre spot only and only in AI-Servo I had this turned on of course) this custom function is set by default to let release the shutter (take the picture) even without focus lock, which means you may not get a focused shot. This is in the user manual the chapter starts on page 301! Past video image processing and so much more, the page you need is 308 but read the whole chapter, as it is all VERY helpful in helping you customise the camera to make it do what you want it to do, the next custom function controls how the camera takes the next image.