Here are the results of a quick noise test I did this evening. I started out trying to make it as fair as possible, but there were a few things against which make this a truly apples to apples comparison.
I set the 6d up on a tripod and used the 50mm f/1.4 lens at f/4. I put the camera in AV mode with Evaluative metering, set a 2 second timer and shot 9 images starting at ISO 100 and doubling the ISO each time.
I then swapped the 6d for the 5Dc and used the same tripod, same lens, f/4 and AV mode with Evaluative metering. By the time I got to ISO 400 it was apparent that each camera had evaulated the scene slightly differently (at ISO 400 the 6D was using a shutter speed of 1/30 whereas the 5Dc was using 1/25. I shot several frames at the camera's recommended shutter speed as well as at the same exposure used by the 6D.
I have also included the 550D in case anyone's interested how that performed. I ended up using the 17-40mm lens at 29mm to provide a similar framing of the shots. This isn't a lens test, it's just a quick look at how each camera handles noise.
What is very apparent from these tests is how important it is to expose to the right. I have included a couple of examples from the 5Dc and 550D of shots which have been taken with 1 stop exposure compensation in camera and then 1 stop underexposed in Lightroom to bring the exposure back in line again. As you can see, this produces a much cleaner image.
I am limited by this forum as to how many posts I can make in one post so please bear with me as I make multiple contributions to this thread adding all the images.
All images were imported into Lightroom, cropped and exported with no other modifications done to them, except for those flagged as ETTR.
First up are the ISO 100 shots from each camera:
ISO 200 images: