APFS vs Mac OS Extended (Journaled) for Time Machine Backup on external HDD
Note this answer now depends on the version of OSX/macOS you have.
This answer is  for macOS Catalina and earlier versions - Big Sur now can use APFS - see  Apple's Types of disks you can use with Time Machine on Mac You can also use HFS+ as before.
You can't use APFS as a Time Machine target:
https://support.apple.com/en-us/HT202784#format
Time Machine drives must be HFS+. That will likely change in the future but currently it is HFS+ only.