DxOMark published their test results for the Canon EOS 6D camera. The sensor performance of the 6D is almost identical to the 5D Mark III. The Nikon D600 still scored significantly higher then the 6D. From DxOMark conclusion:
The EOS 6D offers a well-balanced performance across all three testing metrics, doing enough to put it into 10th place overall on the sensor rankings. Given the price, this does offer very good value for money, matched only by the Nikon D600. It suggests that this new market segment could be a real hotbed of key models for the major manufacturers and is therefore one to watch carefully.