5th gen Extreme would be the sort of Mac Mini shaped one, right? Well, it has more ethernet ports for one thing. And they're gigabit.
The Airport Express would have faster wireless standards and the 3.5mm audio output for AirPlay, but no gigabit.
Never mind, both the 5th gen Extreme and the latest generation Express only support (up to) dual band 802.11n.
The latest generation Extreme added 802.11ac but the Express never got AC support.
So the breakdown:
Express
dual band 802.11n (wash)
2 10/100 ethernet ports [1 WAN, 1 LAN] (con)
1 USB 2.0 port (wash)
1 3.5mm audio-out jack for AirPlay (pro)
Extreme
dual band 802.11n (wash)
4 10/100/1000 ethernet ports [1 WAN, 3 LAN] (pro)
1 USB 2.0 port (wash)