Do you need any of the functions an advanced router has?
If you are not a computer expert you wouldn't be able to work out VPN Tunneling, custom ROMs etc. anyway.
If you have kids, you may like to look into IP or port blocking, scheduling, throttling, scheduling (I am aware that this might be considered as mental child abuse ).
The Airport Extreme is a better choice over the Airport Express, especially if you have newer Mac hardware as it supports 802.11ac, whereas the Express supports the older 802.11n standard.
Both are dual band so you can affectively run 2 wifi networks at home from the same router - one being 2.4GHz and the other 5GHz. (you can have say, the 2.4GHz running 802.11n/g/b for older Macs and Windoze hardware and 5GHz running 802.11ac for new stuff).
Against the usual stuff: malware, viruses, worms, Trojan horses, spyware. Also protect against network attacks, unknown devices, suspicious applications... But I'm only surmising...