DxOMark published the result of Canon 5D Mark III sensor, the overall score of 5D mark III sensor is 81, before jumping to any decision please remember that the test score of DxOMark is based on Sensor RAW output, the low-light score of Canon 5D Mark III is 2293 ISO, Canon EOS 5D Mark III nearly ½ Stop better in low light conditions compared to Canon 5D Mark II (1,815 ISO).
The dynamic range score is almost same as 5D Mark II, the 5D mark III shows its improved performance behavior at high ISO test only.
Canon 5D Mark III vs Nikon D800
On sensor war Nikon D800 is a clear winner, the overall score, dynamic range, portrait color depth and low-light score is better than the Canon 5D Mark III.
for more: Visit DxO Mark Lab
Buy 5D Mark III or Nikon D800 from Amazon