For those interested in real-world noise comparisons of these two 11+ megapixel cameras, here's another data point.
The shots below were taken seconds apart, using the same 50mm Canon macro lens. Both were shot at f/3.5, 800 ISO.
I did a 100% crop from the photos so that you could inspect and compare noise levels and noise characteristics.
Here's the 1DS shot:
Here's the 5D shot:
I used JPEG large on both cameras for this test. Auto white balance, natural light.