There are many theories on the "most bestest" way to lay out your WiFi connectivity. Some are fact, some are fiction, some are a mix of both.
There are generally three things to consider:
- Range
- Performance
- Security
Range
2.4GHz easily has longer range than 5GHz in the majority of use cases. It's ability to penetrate objects with less attenuation helps make it through walls, floors, and furniture more effectively. This is a double edge sword though....since 2.4GHz can penetrate walls easily, this is where the interference from your neighbors comes in since their WiFi can easily intrude on your airspace and vice versa.
Performance
2.4GHz will be more limited in overall speed than 5GHz. Even in clear airspace, 2.4GHz is limited to "N" speeds while 5GHz can get to "AC" and what ever the next faster standards will be. Even in taking the higher link rates out of the picture, 2.4GHz will generally have more interference issues from neighboring devices. These could be your next door neighbors, a Sonos bridge, an Arlo bridge, ZigBee devices, or gobs of other wireless devices that operate in the 2.4GHz band.
Building on this, if you have lower bandwidth chatty devices, you don't always want them on your 5GHz since their constant chatter will take away maximum performance from other clients. So that webcam that only needs 1-2Mbps, but runs all the time, it has the potential to impact your 4K streaming performance.
Security
Some people like to isolate higher risk devices (printers, cameras, IoT, guests, etc) to their own SSID that drops to their own isolated network segment. With this isolation comes trade-offs. If it is a printer or a Sonos or some other similar device, the broadcasts that the device and the client depend on won't be heard. There are ways around some of this...just takes more work.
You will have to weigh all of these options and create a solution that meets your needs. Some people like using different SSID for each band so they can somewhat control which devices are on which radio. Some like using a single SSID to simplify overall setup and roaming.
In my house...I have multiple APs running two SSIDs. One for my primary devices and one for guest. I use the same SSID for both bands and let the multiple APs handle the bandwidth concerns. So even if my wife is streaming her Netflix shows in the Living Room, my laptop in my office still can easily get 300Mbps+ since it is connected to a completely different AP. As I walk around the house, my phone determines which AP has the better signal and switches on its own. Sometimes it ends up on 2.4GHz, but the majority of the time it stays on 5GHz.