you are right, this has been brought up before, but you are new, so i will forgive you. . . this time.
the two big differences i have noticed is that airport extreme has two ethernet connections on it. i used to have one and i had my xbox plugged into the 2nd connection (cable modem into the first). you could use it to plug in an ethernet router to give ethernet network access to other computers/video game consoles on your network.
the airport express only has one ethernet connection. my cable modem is plugged in there.
the 2nd difference is that airport express has airtunes. airtunes is a technology that lets itunes stream whatever song is playing wirelessly to your airport express basestaion, which will play it on whatever speaker system is plugged into the audio jack on it. airport express does not have airtunes.
i think an ideal situation would be to have airport express basestations where ever you have speaker systems that you want music streamed too, and an airport extreme plugged into your cable modem. all of these using wds to join them all into one large airport network.
to sum it up:
airport express:
small
1 ethernet port
airtunes
airport extreme:
bigger
2 ethernet ports
i am sure someone can comment on the airport express and one ethernet port and if there is a way around it. i was thinking of cable modem > ethernet router > airport. not sure if that will work. anyone?