Wireless Bands

What is the difference between 2.4GHz and 5GHz?

These numbers refer to two different “bands” that your Wi-Fi can use for its signal. The biggest difference between the two is speed. Under "ideal conditions", 2.4 GHz Wi-Fi will support up to 450 Mbps or 600 Mbps, depending on the class of the router. 5 GHz Wi-Fi will support up to 1300 Mbps.
Some things to note, First, the maximum speed you might see is also dependent on what wireless standard a router supports—802.11b, 802.11g, 802.11n, or 802.11ac as well as the standard your device supports.
The second thing to note is that important phrase “ideal conditions.” The standard suggests what the device is able to do, not necessarily what the environment around it allows it to do. The 2.4 GHz band is very congested, the band is used by multiple devices and not just for Wi-Fi. Here is a list of Objects that are known interfere with wireless services
Devices known to cause wireless interference with 2.4 GHz band

Old cordless phonesGarage door openersBaby monitorsWireless Thermostats
Alarm systemsMicrowave ovensFluorescent bulbs2.4GHz video cameras
Bluetooth radiosNearby Wireless RoutersPlasma cuttersPower Lines
Car Alarms

Devices known to cause wireless interference with 5 GHz band

5GHz cordless phonesRadarPerimeter sensorsDigital satellite
Nearby Wireless RoutersWireless camerasOutdoor wireless 5GHz bridges 

The 2.4 GHz band is actually very good at penetrating through walls and solid objects, namely due to its longer wavelengths. In theory, Wi-Fi signals are capable of passing through walls and other obstacles relatively easily. However, in reality, some walls are thicker or use reinforced materials such as concrete and may block some of the signals. Materials such as drywall, plywood, and other kinds of wood and glass can be easily penetrated by wireless signals. However, materials such as brick, plaster, cement, metal, stone, and double-glazed glass, antique mirrors may cause problems.
Materials known to block or degrade wireless signals with 5 GHz band

GlassTinted GlassMirrors(Especially Antique)Metal Siding
Insulation(Especially Foil backed)BrickMetalStucco

Which Band should I use?

Which band you use could be different for each person, depending on location, the number of routers near you and building materials. As previously mentioned the 2.4GHz band is better at longer ranges so it’s arguably better if you need better range on your devices, or you have a lot of walls or other objects in the areas where you need coverage. However, because so many devices use the 2.4 GHz band, the resulting congestion can cause dropped connections and slower-than-expected speeds.
The 5 GHz band is much less congested, which means you will likely get more stable connections. You’ll also see higher speeds. On the other hand, the shorter waves used by the 5 GHz band makes it less able to penetrate walls and solid objects. It’s also got a shorter effective range than the 2.4 GHz band. Of course, you may also be able to mitigate that shorter range through the use of range extenders or mesh Wi-Fi systems, but that will mean a bigger investment.

2.4 GHz5 GHz
Longer Range up To 150 feet Indoor 300 feet OutdoorsShorter Range up to 50 feet Indoor 100 feet outdoors
Heavy CongestionLight Congestion
11 Channels23 Channels
Slower Speed typically 54MbpsHigher Speed typically 600Mbps

By the table above the 5GHz band would be the ideal choice in regards to speed, however, its shorter range and problems with penetrating objects, can make it a poor choice. For customers on 5, 15, or 40 Mbps DSL customers would get no appreciable difference as all 3 of those packages are slower than the maximum speed of the 2.4 GHz band. Even customers on 100 Mbps package may not see an appreciable difference between 2.4 GHz and 5 GHz.