Yes, SSDs have massive performance benefits over HDDs when APFS is used, but Time Machine is a special case scenario. IMO, the performance difference really doesn’t matter for most people because:
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TM backups run once per hour, and in the background. As long as an hourly backup completes before the next one starts, who cares if it finished in 5 or 10 or 15 minutes?
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APFS’s ability to create massive fragmentation is somewhat mitigated by the fact that it isn’t randomly modifying files. It creates a snapshot and then writes the files that changed. So changes tend to be localized near each other. Even though periodic snapshot deletion (hourlies after 24 hours and dailies after a week) does create fragmentation, it’s not going to be nearly as much as you might see on an HDD used as a boot device or for document storage.
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The price difference is likely to be more important to those of us on a budget (almost all of us), especially when the use-case is backup and not for working files.
For example, a 2TB SSD costs $130-280 (from MicroCenter). Looking for an external HDD for under $150, I found a 6 TB drive). Looking for an external HDD for under $280, I found a 16 TB drive).
The ability to get 3-8x the capacity for the same price, especially for an application where performance is not critical, is not something you should ignore when making purchasing decisions. (Of course, you should still research each products’ reliability before making a purchase and not just look for the lowest price/TB ratio. This is true for both SSDs and HDDs.)