Running Continously or Allowing HDD To Spin Down

Windows 10 Pro x64

I have two HGST 4TB drives in USB 3.0 External Docking Stations. They appear to the operating system as SCSI disk drives. They are used strictly for backup which runs once between 4:00 AM and 6:00 AM. I was concerned because the drives were on continuously even through they weren’t used most of the time.
I found a setting in power management that will turn off the drives after 5 minutes of inaction. That means they will be off most of the time. However, starting explorer will cause the drives to spin up. It also delays explorer from opening for 10 to 20 seconds.
My question is it better to have the drives spin down and stay off for most of the time even though it means that may be spun up a number of times during the day or would it be better to just leave them spinning all the time?