It depends. There are a variety of factors that will dictate which band your devices should utilize.
Lots of devices in today’s world use the 2.4GHz wavelength, including microwaves, baby monitors, and garage openers. While you may not have many of these devices in your home, if you live an apartment surrounded by other people with their own routers, the 2.4GHz band is likely congested with all sorts of devices. If you have a device that doesn’t move around and will be located near your router (and you can’t connect it with an Ethernet cable), we recommend having it utilize the 5GHz wavelength to reduce congestion and take advantage of the higher speeds that the 5GHz wavelength can provide.
If you have a device that moves around your home a lot (like your cellphone) and it is usually located farther away from your router, we recommend you set this device up to take advantage of the 2.4GHZ wavelength. The 2.4GHZ wavelength has a larger range and can penetrate solid objects easier than the 5GHZ band, making it ideal for devices such as cell phones or laptops that are taken from room to room or are located farther from the router.