Don't disable the 2.4GHz band on your router as any older wireless devices that are not dual band (ie the device can only use 2.4GHz) won't be able to connect to it.
Using the 5GHz band costs no more then using 2.4GHz. The advantage of using 5GHz is that it's much less crowded than 2.4GHz which most people use, so interference from other routers in your neighbourhood is less likely to occur.
One downside however is that 5GHz has a shorter effective range compared to 2.4GHz, so over a given distance a 5Ghz signal will be weaker. For that reason, many people prefer to use the 2.4GHz band even if they have a dual-band router.