The additional antennas required and the additional stream also fill in hard-to-reach spaces with better Wi-Fi coverage. For larger homes or offices, or ones that have dead areas with an Express or other Wi-Fi gear, the Extreme or Time Capsule may be required.
(Image Caption: Ports: (left to right) Power, ethernet WAN, ethernet LAN, USB, and audio (analog/optical). ) The AirPort Express once had a single ethernet port, making it useful only to extend networks via ethernet if it were plugged into a broadband modem. Older models could also extend an existing Apple wireless network (that feature still exists) and use the single ethernet port to provide access to wired machines. The new second ethernet port opens up the Express by allowing both a Wide Area Network (WAN) connection to a broadband modem or a larger network, and a Local Area Network (LAN) connection to a computer via ethernet, or to an ethernet switch to which many computers and devices can be connected.
The Time Capsule and Extreme each have a built-in three-port gigabit ethernet (10/100/1000 Mbps) switch, while the Express continues to use 10/100 Mbps ethernet on both its WAN and LAN ports. However, the 100 Mbps limit doesn't affect data exchanges between Wi-Fi devices, which work at the maximum wireless rates available, and it's easy enough to plug in an inexpensive gigabit ethernet switch to allow the fastest throughput between wired devices.
Only two particular situations require an Extreme or Time Capsule because of the ethernet limitation: if you have a broadband connection of faster than 100 Mbps, which is uncommon, but becoming more widely available; or you are determined to have the maximum possible throughput between ethernet and Wi-Fi devices on the same network.
Guest networking has also been added, which allows a second virtual network with a unique network name (SSID) and security scheme to be available to visitors or others without providing access to the main network. Guest users can't sniff or have access to Bonjour resources (like shared printers or file servers) or other traffic on the main network, either. This has been in Apple's full-sized base stations for years, and it's a nice feature to find in the $99 model.
Apple also brings over the option from the other simultaneous dual-band gateways of naming the 2.4 GHz and 5 GHz networks separately. That lets you choose to have a device connect to one band or another if it has the option of connecting to either. On some combinations of devices and networks, it can be useful to force 5 GHz for throughput or 2.4 GHz for range.