Luminous-Landscape compares the Canon 1Ds MKIII VS Nikon D300 VS Nikon D3 and write: “Sad to say, D300 is not in the same low noise league as either the D3 or the 1Ds MKIII. Noise is more apparent, and though the exposure was 1/50th second vs the D3’s 1/60th second (essentially identical), the image was overall quite a bit darker, indicating that the ISO was likely not what the camera indicated it was; maybe ISO 1000 in reality, and the apparent dynamic range somewhat less as well. The D3 Nikon is now back in the IQ game. Canon has enjoyed a deserved reputation for high image quality, especially at high ISO. But now with the D3 Nikon has upped the ante, producing a camera that appears to offer lower noise than Canon (at least against the flagship 1Ds MKIII), and higher available speeds as well.”
Read more about the Canon EOS 1Ds Mark III