The Express doesn't have this new feature that the Extreme and Time Capsule were just updated with:
Simultaneous dual-band support.
Some Wi-Fi devices use the 2.4GHz wireless band, including iPhone, iPod touch, and devices using 802.11b/g. Other devices can use either 2.4GHz or the higher-speed 5GHz band, such as the latest 802.11n-based Mac computers and Apple TV. Instead of choosing one of the bands, AirPort Extreme now operates simultaneously on both bands, and your multiband devices automatically use the best available band. This means all your Wi-Fi devices get the fastest possible wireless performance and the best possible range.