I have used the 20D for most of my shooting and the 1D classic occasionally when the resolution was not important. Most of my shooting is family (my kids) and birds (during hikes around the bay area).
Due to a chronic elbow injury, I am now forced to lighten my load for most shooting and as a result I have decided to downgrade to XTi.
I am wondering if any of you have done this transition what I should expect. I expect to have the XTi next week and any pointers / suggestions would be great.
I've used both the 20D and the XTi extensively. Both have their areas of inferiority and superiority. I am only going to mention things here that are likely not to be mentioned by others.
The XTi viewfinder is a little darker, but it is much *sharper* than the 20D's, a plus if you use manual focus at all, or like to double-check the camera's AF. The AF on the XTi is newer-generation, too (same as the 30D).
The XTi, of course, has more pixels, and is potentially better for overall image detail. This comes through all-around with good optics at ISOs 100 through 400, but doesn't come through in the shadows as well at ISO 1600, because the higher ISOs on the XTi have more noise, relative to absolute exposure; about 1 stop more at ISO 1600. ISOs 100 and 200 on the XTi have slightly more dynamic range than the 20D (and the 5D as well, at ISO 100). There's a caveat here, though. The XTi by default exposes the RAW data about 1/2 stop lower than the 20D, so you need exposure compensation of about +1/2 EC more than with the 20D, to make full use of the low noise at ISOs 100 and 200; otherwise, the extra DR only goes into increased highlight headroom, and no signal-to-noise advantage over the 20D.
I find that with the XTi, and my 100-400 IS with teleconverters of 2x or greater, the mirror slap on the XTi makes the IS react irrationally, shifting the composition down 1/2 frame, but not always, so it is impossible to compensate with 100% certainty.