The optimal in performance is that all devices have the information configured locally. But this is typically more work to maintain so a compromise is to make use of your Airport express ' ability to act as a DHCP server. A DHCP server is a convenient way to specify things like IP addresses and DNS server IP addresses in a single spot. From here this info will get propagated out to any device that connects to your Airport and asks for a IP address via DHCP.
When computers connect via DHCP the info they're given is kept locally there after for a certain period of time. This info is essentially leased from the DHCP server, so the clients are guaranteed that no other system on the network will be given this same info by the DHCP server.
With respect to which DNS server IPs to use, it really depends on which ones are geographically closest to you both in terms of physical distance from you and also how far they are from you from a network perspective.
This is probably the best tool for determining what DNS server you should use.