The 6D displays excellent JPEG processing and noise reduction, as well as generally low noise. JPEGs are pretty clean up to ISO 800 and good up to ISO 1600. JPEG images can be usable as high as ISO 25600, though only if they have little or no detail.
Note that the shutter couldn't operate fast enough for a proper exposure at ISO 25600.
These images are roughly comparable, but were shot months apart with different settings. Still, you can see how the Nikon JPEG settings err on the side of more detail/more noise compared with Canon's less noise/less detail approach.
(Canon: 1/100, f2.2, AWB, ISO 1600, spot metering, Canon 50mm f1.2 lens; Nikon: 1/40, f4.2, AWB, ISO 1600, spot metering, Nikon 24-85mm lens at 52mm)
Canon's JPEG processing is excellent. Although I could get a little more detail and sharpness by processing raw, the JPEG version isn't bad. I was able to get prints of detailed night scenes from ISO 1600 JPEGs that displayed few artifacts.
While the Auto (default) and Standard Picture Styles still push contrast and saturation a bit too far, they don't seem as bad as I've seen from previous Canon models; reds are a little too deep, but the hue shifting isn't as bad as I've seen.