I was catching up on podcasts while on vacation and listened to Mac Geek Gab 453 where Dave Hamilton discussed his interview with Alf Watt at WWDC. (The discussion is about 40 minutes in) Alf is the developer of iStumbler and for many years worked for Apple in their Wireless Networking division.
In the interview, Dave asked Alf a question I always wondered about. If you're running a dual-band access point, such as the newer Airport Extreme or Time Capsule base stations, using 2.4 GHz and 5 GHz bands, should the SSID for 2.4 and 5 GHZ networks be the same or different? Alf's answer - the same.
When setting up my Airport Extreme years go, I choose to segregate my 2.4 and 5 GHz networks because at that time many of my devices, would not support 802.11n, much less the 5GHz band. While the first and second generation models of the Airport Extreme would support either 2.4 or 5 GHz bands, they would not support both at the same time. Dual-band support didn't come until the third generation Airport Extreme and second generation Time Capsule in 2009. For this reason, I choose to segregate my networks by selecting a different SSID for my 5 GHz network. As I've upgraded my tech, I kept the same network configuration, until recently when I gave it a second look.
Today, base stations support simultaneous dual-band operation and the newest iOS hardware has been updated to work with 802.11n in either frequency. Assuming you have newer devices, for most people the best approach is going to be to setup a network that does both 2.4 and 5 GHz as a single network and let your devices dynamically switch based on the best signal availability. For me, this meant launching Airport Utility and unchecking the box to create a separate 5 GHz network. If you have any devices set to only connect to the 5 GHz you'll need to reconfigure them to connect to the primary network.
ArsTechnica has an article explaining the various technologies in more depth and how to setup your network based on your circumstances. The Mac Observer's interview with Alf explains the reasons why you want to let your devices dynamically switch networks for the best performance.
After a few days running the single network all my devices are dutifully switching bands when appropriate I'm happy to have have one less network to manage.