The 5D is a more professional camera. Your image quality will be better, but only if you are shooting with good glass. I often see a lot of dicsuusion about the bodies, but not the lenses. The acutance (sharpness) and abberration (color) are in the GLASS.
A 20D with L-series glass will take better pictures than a 5D with a $200 Sigma zoom lens.
Full frame means the sensor is the same size as a piece of 35mm film.
Previously, most sensors were smaller, so when the image was reflected on to the film plane, the sensor would only capture the middle part of the image where the sensor actually covered.
Because the edges were cut off (not in the viewfinder though, the viewfinder showed an accurate representation up to 97% usually), it made the subject appear closer than it was. This created a "zoom effect."
This could be a factor of anywhere from 1.8x to 1.3x depending on the camera. The 20D has a factor of 1.6x, so you would have to multiply the focal length of your lens by 1.6 to get the "real" focal length.
All that being said, I found that depth of field suffered from the magnifciation and I finally have it back on my 5D. However, unless you have good glass, it doesn't matter.
