Bob Atkins has done an extensive comparison of the merits of the Canon fullframe 5D vs. The Canon APS-C 20D.

The conclusion is that the 5D wins the comparison but with some caveats.

It's a win, but not a total "slam dunk" for the full frame sensor, at least as far as the EOS 5D vs. EOS 20D goes. With good lenses the results from the EOS 5D are better, though with noticeably higher vignetting when lenses are used wide open. Of course you can mostly compensate for vignetting in software, but still you may lose something. With low cost lenses shot wide open, the full frame advantage disappears, and in situations where you have your longest lens in use on an APS-C camera, switching to a lower pixel density full frame camera and cropping will lead to lower resolution images. [ more ]