Perhaps those with a keener eye than my own will provide insight into what we're seeing. My thoughts are that the noise is different, that's all. Noise I see in both pictures. This is definitely "pixel peeping," but I felt it worthwhile to see just how "bad," or not, the 1DS was at ISO 800 compared to the 5D.
Here are comparison shots at ISO 1250:
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.