Does it make sense to keep different SSIDs for 2.4GHz and 5GHz wireless networks?

Most wireless stacks do not consider these networks to be different to each other, so 2.4GHz has the same weighting as 5GHz.

If this isn't important to you, then keeping the SSIDs the same will mean it will pick whichever it sees first.

If you keep the SSIDs different, it means that you can prioritise 5GHz over 2.4GHz by adding both to your Wi-Fi connections, and saying that one is better than the other.

Note that 5GHz is not inherently faster than 2.4GHz. They both have the same theoretical maximums, 150 megabits per second (single radio chain), 300 megabits per second (two radio chains and two spacial streams), or 450 megabits per second (three radio chains and three spacial streams). However because the 5GHz frequency band is less crowded, there is more chance that the AP can get a full 40MHz band of radio to carry data.

If you have high quality Wi-Fi client devices, it's best to use the same SSID for both bands so your clients will automatically roam to the band that suits their needs best.

If you have low-quality Wi-Fi client devices, you might need to second-guess their band-choice decisions, so you might want to have separate SSIDs.

Your MacBook Pro should not have shown you two networks with the exact same name, unless you had accidentally configured two different security types for the two networks. Or maybe you thought you had configured the exact same name for both bands, but you'd accidentally put a space at the end of one of the names and didn't notice.

Your iPhone 4 doesn't have a 5GHz radio, which is why it can't see your 5GHz network. I suspect the same is true of your HTC Desire HD.

I recommend that you leave your 2.4GHz network set to 20MHz-only. Using 40MHz in 2.4GHz doesn't leave enough room for other uses of the band, such as Bluetooth. All of Apple's N-capable gear limits itself to 20MHz operation in 2.4GHz (even if the third-party Wi-Fi AP is configured to allow 40MHz operation in 2.4GHz), in order to leave room for Bluetooth. So your MacBook Pro is only going to use 40MHz-wide channels in 5GHz.

I think I found a solution for Androids that prioritize the 2.4 GHz Wi-Fi band over the 5 GHz band with the same SSID if you access a dual band router at home for example. If both bands have the same SSID find out which band your Android device is currently connected to using the free inSSIDer app. My was connected to channel 6 instead of channel 161 (my desired 5 GHz channel). Move to a location where you can confirm that you are connected to the 2.4 GHz band (hit the refresh button as needed on the top of inSSIDer to update) When you verify that you are connected to the 2.4 GHz band, go back to your Wi-Fi settings, you will see your SSID twice with the top one being the 2.4 GHz connection select that and select Forget.

This is better than having to choose the 5 GHz band only setting in advance settings as there will be many other places where you need to connect to both bands. Now I get great Wi-Fi speeds at home instead of competing against the crowded 2.4 band at home. Hope this helps.