What do you want one of them for because they provide different features?
The AirPort Express connects to your router via a 100 Mbps connection and provides one outgoing 100 Mbps port. It is a wireless access point to which you can connect devices wirelessly and it also supports Apple AirPlay. It supports both 2.4GHz and 5GHz WiFi. Although no longer sold by Apple, they did recently issue an update to its AirPlay feature.
The Airport Extreme connects to your router at 1Gbps and has three outgoing 1Gbps ports. It also supports both 2.4GHz and 5GHz WiFi, but doesn't have Apple AirPlay.