I upgraded my FF from a 5Dc to the 6D. If low-light was your motivation, prepare to be surprised. This shot was taken taken with a 6D/Tamron 28-75 at 75mm. ISO 102,400 - 1/20, f/5.6. Lighting is a 60W incandescent spot bounced of the ceiling at camera left, and a 40W fluorescent fish-tank lamp directly camera right. It isn't art, but it is useable.
This is not SOOC. I had to play with it more than I care to in LR. There was lots of yellow noise so I had to de-saturate yellow by 25.
I think 102,400 is a bit much, as in emergency only. ISO 25,600 jpgs are fine right out of the camera. I am experimenting with auto-ISO with 25,600 as my upper limit. Noise is just not much of a problem for this camera. I can compare it to my 60D as follows. Since I shoot raw, I do run through the DETAILS panel in LR for NR/Sharpness. I find the 6D files at 25,600 require about the same level of attention as my 60D does at 3200-6400 depending on exposure and subject matter. An easy two stops of low-light improvement.
I don't shoot anything that requires 25,600. My new 35 f/2 IS and the 6D at 1600-3200 is all I need. I puts the 5Dc to shame at those levels. And more is available if I need it. Enjoy your new camera.
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Bodies: 60D, 6D.
EFs: 15-85, 10-22
EF: 28-75, 35 f/2 IS, Σ70-200 OS, 100-400L
Flash: 580EX II, 430 EX II