Interesting experiences there, Roy. "Interesting" because my experimentation with a very similar pair of cameras leads to a very different conclusion!
I had two 20Ds, which were pretty much indistinguishable quality-wise. Naturally, I had a favourite one out of the two, but I don't try to fool myself that the older one was in fact any better than the newer one. I still have and use the older one, on its second shutter now and (usually) demoted to wide-angle landscape duties - mostly because it hasn't got a self-cleaning sensor and I tend to leave either the 10-22 or hte Tokina 10-17 fish on it all day. I sold the other 20D.
I have also owned a 400D and two 40Ds. Nothing to choose between these three either, except that I missed the back control wheel on the 400D and believe that it got exposures wrong more often than the 20Ds and 40Ds. But assuming correct exposure, it was much of a muchness detail-wise with the 40Ds. I gave the 400D to a family member and still have both 40Ds, though I'll give the older one to a different family member soon. (For no reason at all, I like the newer 40D better than the other one. In reality, I'm sure that they are exactly the same. But sometimes I pretend that the newer one takes better pictures. Well, right now it does because the older one needs a service, but usually they are the same.)
Anyway, that gives me 5 cameras in the Canon 8 and 10MP families, and I have never been able to make any real or meaningful distinctions between them. Even when I shot some pretty careful formal tests a while back, there was nothing in it. Maybe marginally more detail in the 40D shots - and I do mean marginal - but more pleasing colour and contrast in the 20D shots. Bottom line: the only things I miss about my 40Ds when I'm using the 20D are: (1) raw buffer, (2) faster shot review to check exposure when you chimp, (3) ISO in the viewfinder, (4) sensor cleaning, and (5) pre-programmed user modes. Want ISO 100, mirror lock-up, f/11, and 2 second delay for tripod landscapes? Turn the 40D to USER 1, take the shot. Want to go back to standard hand-held general settings? Return to Av mode on the dial and it remembers my ISO 200, single shot, whatever else I want. It's a great feature!
But IQ-wise? I'd just as soon use the 20D.
I look pretty carefully at my shots and have no reason not to believe that you do the same. So why do we come up with such different answers?
(I'll turn to your question in another post.)