I found iso performance on the IIN to be significantly better than the XT: ISO 640 on the 1DIIN is about like ISO 400 on the XT, ISO 1250 on the 1DIIN like ISO800 on the XT. I was satisfied with the ISO 1600 performance, though it would look noisy viewed on a big monitor. My guess is that the 50D would have the edge in high iso performance. The 50D also af is rated to work in half the light of the 1DIIN (which in turn is rated to work in half the light of the XT autofocus).


For low light performance (both low noise and low light autofocus), my guess is XT < 1DIIN < 40D or 50D. If that was my primary concern, I would go with 40D or 50D. For action, I would pick the 1DIIN. The 1DIIN is super responsive.