By Tesserax:
The important thing to understand about the new dual-band AirPorts is that they employ two (2.4 & 5 GHz) independent wireless radios that, by default, share the same Network Name or SSID. These radios operate simultaneously but are still independent.
Also, by default, the dual-band AirPorts will attempt to configure both radios to be as compatible as possible to a wide variety of 802.11 wireless clients. So for the 2.4 GHz radio, 802.11b/g/n clients can connect, and 802.11a/n clients for the 5 GHz radio. Note the "overlap" in "n" clients. That is because "n" clients can operate in either the 2.4 or 5 GHz band ... both not both simultaneously.
Most "n" clients will attempt to connect to the radio with the strongest signal ... but this is not an exact science. "a" clients will always connect to the 5 GHz radio and "b" & "g" clients will always connect to the 2.4 GHz radio. So, depending on how you configure the AirPort's radio modes, you can have either a wide variety or a narrow one of wireless clients that can attach.
For best bandwidth performance, you will want to do at least two things:
1. Set the 5 GHz radio to only allow "n" clients to connect, and
2. Set the 2.4 GHz radio to only allow "n" clients to connect.
This would prevent non-"n" clients from connecting and degrading the overall bandwidth of the radio they are connected to.
Ok, but what, like in your case, if you have non-"n" clients? My recommendation is to set the 5 GHz radio to the "802.11n only (5 GHz)" mode and the 2.4 GHz radio to the "802.11b/g/n" mode. Also, if you want to "force" "n" clients to the 5 GHz radio, select the option to create a second Network Name for that radio and point your "n" clients to it.