Main difference is that 480p should not flicker.
This is due to the way new flat screens work differently to CRT. As the CRT is a single beam that scans the whole screen in a pattern when the beam reaches the bottom of the screen the top will have dimmed slightly.
In flat screen all pixels are refreshed simultaneously and so they should stay at constant brightness.
Only the new hdtv cables such as component and hdtv allow for progressive (480p) scanning, also a CRT is incapable of prog scanning