For 2.4GHz/5GHz wifi which is best for Apple devices: different SSIDs or same

You can change the system preferences for JoinMode and JoinModeFallback to be the following:

    JoinMode (String)
        Automatic
        Preferred
        Ranked
        Recent
        Strongest
    JoinModeFallback (String)
        Prompt
        JoinOpen
        KeepLooking
        DoNothing

Do this using the airport command:

/System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport

Run the command to see the options, up the top you'll see a section on how to sec preferences.

For instance:

sudo /System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport prefs joinMode=Strongest

Note, this preference isn’t permanent, so you may need to automate this or issue it for specific problem situations or network providers.

You can not tell from the iPhone directly (as of iOS version 7 native). You should be able to tell via the wireless router your connected to in it's logging functions, WiFi client list or some similar mechanism within the router. I can't provide you an exact process as every router is different. This answer assumes you do not want to JailBreak your iPhone. If you are not adverse to doing so there may be an app in the Cydia app store that will show more meaningful information with regard to WiFi frequency information.

You can however detect what cell tower frequency your phone uses by using Field Test mode. This will special mode allows users to find out a range of information about the phone’s connection to the network. In particular, the information displayed in the Field Test mode allows users to discover exactly which tower their phone is connecting to and which frequency band (WiFi) the signals are being sent on.

To access this mode:

• Access the iPhone's Field Test mode by typing *3001#12345#* in the phone’s keypad and press dial. This will bring up the Field Test menu.
• Select the “Cell Information” menu option to display a summary of all nearby towers. The “RX” code displays the signal strength of each tower. These RX values are dBm signal strength readings, where lower values means strong signal (so -60 signifies stronger signal than -80). The towers are listed by signal strength, and the top tower is usually the tower your phone is connecting to.
• Selecting the first tower will take you to a further menu that displays more specific information. The “Freq” value shows the frequency block that the network is using. If the Freq number is between 128 and 251, the tower is using the 850 MHz Cellular band. If the Freq number is between 512 and 810, the tower is using the 1900MHz PCS band.