Is there a reason to not choose APFS?

Personally I would use MacOS Extended for an external data drive. APFS is too new of a file system which still has some growing pains, plus First Aid is unable to repair lots of APFS file system issues. When First Aid is unable to repair an APFS volume, the only option to fix an unrepairable APFS file system is to erase the drive and restore the data from a backup. Whereas with the HFS+ (aka MacOS Extended) file system, if First Aid is unable to repair HFS+, then you have an option to use the paid app Disk Warrior to repair the HFS+ file system. Disk Warrior has proven itself over decades, that it can repair almost any HFS+ file system issue as long as the drive is not physically failing.

Unfortunately there are no third party apps which are able to repair an APFS file system because Apple has not released the necessary APFS documentation. In theory, the APFS file system should be safer for the data if a save is interrupted, but in my own personal experience I have encountered too many instances of APFS becoming damaged on our organization's Macs and needing to have the drive erased and restored from a backup or clone.

If you have older Macs that may need to access the external drive, then HFS+ is the logical choice for a data drive.

babowa, the reason for my initial comment is that Sierra and earlier cannot read APFS. Later OSes (like your HS OS) can read Mac OS Extended (Journaled), so it is actually more "universal."

Hi,

If you might be using the T7 with Sierra (10.12) or earlier, choose Mac OS Extended (Journaled).

▷ Protect your Mac information with encryption. 

I'm using Mac OS Extended with High Sierra (10.13) on my 2017 iMac and all my externals (multiple clones and drag 'n drop). Works fine.

Ahh, got it.