The Fritzbox comes with 2.4 and 5 set to the same SSID, at least mine did. After a bit of reading, it appears that if you have 2.4Ghz and 5Ghz set to the same SSID, then devices should in theory roam to whichever is "best" in the current environment, but that this doesn't always happen in practice and some devices will just connect to whichever is found first, or just default to 2.4Ghz all the time. 

In my neighbourhood (as many others I would guess) 2.4Ghz is fairly saturated whereas 5Ghz is pretty clear (at least according to wifi analyzer on my phone)

I changed my fritzbox to have different SSIDs so I can explicitly connect to 5Ghz and seemed to get a decent improvement in throughput to my Surface pro (v1) which is what I use fairly often over wifi (most other stuff in my house are wired)

I'm curious on other peoples thoughts on whether it's better to have them set to the same SSID and rely on devices roaming as needed, or is this unreliable in general?