Do you care primarily about performance? HFS+ was designed for spinning rust, APFS for SSDs. APFS’s CoW model will lead to more fragmentation, especially on a smallish disk.
Do you care primarily about filesystem integrity? HFS+ has a bunch of bolt-on fixes for a floppy-disk filesystem, APFS has checksums and the CoW design makes it more fault tolerant in the event of power failures or abrupt disconnection.
Also note that Apple has surfaced the CoW features of APFS to a bunch of userspace tools, so these will fall back to doing real copies on HFS+. HFS+ also lacks support for holes, so if you are putting things like disk images or other files with large runs of zeroes, APFS will lazily allocate the space as needed, HFS+ will write a load of zeroes to the disk.
