2.4 GHz is an older standard and shared with cordless phones and other devices. There tends to be more congestion on 2.4 GHz, but it also better going through walls.
5 GHz is a newer standard and can carry has higher bandwidth (faster). There is less congestion and more channels at the 5 GHz setting, but it is less effective at going through walls.
Which should you use. If you can get a good connection at 5 Ghz, I would use that. It has the potential to be faster. If your connection is dropping out or lagging, try 2.4 GHz. It's slower, but more reliable.
**edit** Some devices (phone, printer, wifi adapter, etc) don't support 5 GHz. If that is the case with your devices it could limit you to 2.4 GHz (or looking for an upgrade!).