Does it make sense to format an external HDD as APFS?
APFS gives several benefits over exFAT, for example:


Protection against meta-data corruption caused by for example sudden power loss, system crash or if disconnect the external hard drive without unmounting it first. exFAT only detects corruption using checksums, but cannot rollback like APFS.
Better support for full disk encryption
Allows snapshotting the file system (i.e. you can create "frozen" versions of everything on disk, which will never change - even when you continue to alter the contents of your files)
Allows cloning a file into two copies that can be independently changed while only requiring the disk space for one file plus the size of the differences compared to the other file


On the other hand, exFAT has a range of advantages too:


Older file system that is "tried and tested" for years on macOS
Simpler file system with less overhead
Most probably slightly faster (but this will depend on the final implementation in the release version of High Sierra)
As you mentioned, the drive can be used without extra drivers on Windows


Only you can decide which set of benefits are most important for you.