Both are suitable.
Express - Ethernet WAN port, attach to your router.
802.11g support
Support for "Airtunes" speakers
Support for USB shared printer
Extreme - Ethernet WAN port, attach to your router
Four Ethernet LAN ports for local devices. 100BT (early model), 1000BT (current model)
No "Airtunes" speakers
Support for USB shared printer
Support for USB shared hard drive
Some other subtleties.
This is the quick answer, you'll get more specifics from others.