based on my experience of owning the current 580EX(apart from the faster cycle time and weather seal on 580EXII should be the same la
) and preowned 420EXII:
580ex more power (GN 58 vs GN 42)
580ex faster cycle than 420exII
580ex can twist 180 degree to left or right and 7 degree down(usefull for very close up shoot) while 420exII can oly twist 180 degree to the left and 90 degree to the right(some time is a bother when i'm shooting on portrait with my DIY bounce card)
580ex got strobo flash function, 420exII no
580ex can act as wireless master while 420 exII no
580ex can use external powerpack that can further enhance the recycle time and safe critical time from changing the battery, no external battery slot for 420exII
now u decide