I only owned my 10D for 4 months and I owned the 20D for two of those
months. The 10D is a very capable camera but the thing that made me
change was the startup time as I shoot mostly wildlife. The delay at
startup once the camera went to sleep didn't work for me with the 10D.
I feel the 20D has slightly better color accuracy and contrast, less
noise at all ISO speeds but really noticable above 400 ISO.
I don't find the AF speed to be much different and the buffer and write
speed are better with the 20D. I also like the overall look of my
images with the 20D better but I did take some excellent images with
the 10D. There are people with more experience with both bodies but
these are my observations.