2.4 GHz network is the most common used, its signal coverage is better distributed on horizontal line floor and it could be easily extended/amped using proper network tools.
5 GHz network has less signal coverage than 2.4 GHz one, but it’s suggested for high data traffic (hi-quality audio stream). It works well only if the 5 GHz unit is near to the router (suggested in particular for the wireless devices of Multiroom groups).