With High Sierra, spinning drives and Fusion drives are not converted to APFS, only SSD's are, and you are never given the chance to choose an option. With Mojave, all types are converted to APFS, and Apple's current version makes it very slow on spinning drives, mostly due to fragmentation.Tom, I do know that when you install High Sierra onto a blank hard drive for the first time you have the option between choosing HFS+ or APFS. This holds true and the same for an SSD drive. [See below –MacInTouch]
Since I did not go this route, I’m not sure what you will see when upgrading or using a combo installer for High Sierra. Of course, no matter what, do a full backup of your hard drive before taking the next step.
If you have another external drive, you can clone to it, reformat your internal drive to HFS+, then clone back – this even works for Mojave. Just be aware that, depending on your Mac model, you may get a warning when trying to update OS items that it can't update, because it requires APFS, creating a Catch-22 problem (especially for T2-based Macs).
My recommendation on spinning drives: do not install APFS, or you will suffer a performance penalty. Also with APFS formatted volumes, Apple's Disk Utility First Aid is the only one that can check an APFS volume for problems. No third-party app yet can work on APFS-formatted volumes including disk defraggers, which on SSD's is only required for Adobe Photoshop or Illustrator, due to their Scratch disk space usage requirements.
One final recommendation, backup, backup, backup, especially with APFS volumes.