Same or different SSID for 2.4GHz and 5GHz ?
Some simultaneous dual-band wireless routers and newer 802.11ac devices offer the option to use the same SSID for both the 2.4GHz and 5GHz bands.
Clients that are capable of connecting to either frequency will typically connect to whichever network they see first.
While it is a personal choice, and you can set them both the same, we prefer to use two separate SSIDs, with names that hint the frequency.
Using to separate SSIDs for the bands allows for people to choose the expected best network, instead of client devices making the choice automatically. In our experience, some older client devices are not capable of choosing the better network for their location automatically. Also, one can distribute client devices manually among the two frequency bands to group them intelligently depending on distance, signal strength and bandwidth requirements.
Note that if using multiple routers/APs, it is still the best practice to use the same SSID, security type and passphrase across them. This is not to be confused with using the same SSID over different frequencies.