A new 20D would be more durable. The 20D has a better viewfinder (prism vs. mirror with .9x magnification vs. .87x). The 20D has a higher top shutter speed (1/8000), higher top ISO (3200), higher frame rate (5 FPS vs. 3.5). The 20D has a rear control dial.
In almost every other way than that (image quality, LCD...), the XSi is better given it's newer technology. It's also no doubt newer, right, and that throws out the 20D's advantage on durability if it's been used longer.