Thanks for the information, Adora and Fishrrman.
I searched and found other sources that indicate HFS+ is better for HDD. That HFS+ is better than APFS for HDD is a surprise to me, as the documentation for Carbon Copy Cloner 6, which I use, says to choose APFS.
Interesting that they in particular say this given this blog post:
bombich.com
The issue is an interesting but not uncommon one over extended technology cycles. As we transition from one technology to another, what was once an optimization becomes a bottleneck for the next technology. We saw this with tape to disc to solid state.
Optimizing for tape access was critical when seek times were seconds to minutes. People bought 3rd party tools to sort their data. Also less common to put lots of unrelated data on one tape.
With disk drives, "rewinds" are much less of an issue and with more capacity so letting users put every unrelated thing on one filesystem is much more user-friendly. However, the mechanical nature of the devices still means seeking small blocks of data can still dominate data access. Filesystems of times past went to great lengths to keep files' data blocks optimized for long sequential access. Performance differences could be orders of magnitude.
With SSD, the cost of seeks is *almost* neglible. It's still better if data is sequential but not worth going through historical heroics to optimize (e.g. various reordering and reallocation of data, more complex data structures to minimize seeking, etc).
As another example of this, MS implemented a new data sorting algorithm with SQL Server 2016 that was optimized for large memory systems and SSD. However, while the new algorithm is somewhat faster for modern target servers, it is painful to sort datasets larger than the server's memory on HDD. I recall one time I had to keep canceling a query for an analysis I thought would take a few minutes because it didn't come for hours. It was very confusing until I found someone blog post about a flag that would tell MS SQL to revert to the old algorithm.
In any case, I am not sure how much better HFS+ is than APFS on SSD but Apple is clearly going that way and so using HFS+ on SSD seems like a recipe for future conflict. On the other hand, HFS+ is so much better optimized for HDD than APFS that I can't recommend using APFS there even in the short-term (for general use, not counting special cases like Time Machine, etc and there I can't speak either way to whether an HDD with APFS to run Time Machine makes any sense).
