Most users should use the default setting of having both bands use the same SSID, since devices will automatically connect to the best signal quality based on their capabilities and location in relation to the router.
Assigning a separate name to the 5 GHz band is a way to try to "force" a computer to connect to 5 GHz.
This would really only make sense to use if you found, for example that in your normal location for a laptop or desktop, that it could not decide which band to connect to since things were about equal.
In a case like this, the computer would always connect to 5 GHz since it was "pointed" at that network.
If you ever decide to add another Express to "extend" the network, you must configure both Express devices to have both bands use the same wireless network name, or SSID.