Yes, I understand that the potential methodologies for this sort of comparison are complex. I think I have done it correctly to make a fair comparison, but I'll be very pleased to be corrected if there is a better way.
Let us say that my goal was to make the best 552 x 368 image I can of this distant kingfisher for my website. For the 40D image, I simply cropped it to 552 x 368 and tweeked a small amount of unsharp mask in Photoshop, and it was done.
For the comparative 50D image, I cropped exactly the same area, which works out to, yes, something like 700 x 466 (I forget the exact figues now) and tweeked a small amount of unsharp mask in Photoshop, and then finally resized it for my website to 552 x 368.