Both methods are digitally signed with the same certificate, and checksums are part of the Cert. They have to be identical. Preparing likely expands/decompresses the update and merges the deltas into the whole, the certificate is verified against the end result. Both methods should produce 100% identical install images.
During upgrades, install scripts run to install services, migrate preferences and run other migration and set up tasks. Sometimes they get errors when they run (especially during beta cycles when they’re still testing them). There could be coding errors, errors in asynchronous updates (parallel updates dependent on one another) and general timeouts (something never finished in time).
If you get errors in applying an update, a reapply may solve it. There is only one way to reapply: IPSW.
Neither way is superior in applying that first try, there’s only one way to do a second try, and if you often feel you have to do this more than once, you will easily feel justified in downloading the big IPSW image once, rather than downloading OTA then IPSW. Makes enough sense.
For me, I do OTA and if I need to reapply, I then download IPSW, which is extremely rare.
You mean like Apple Pay? Took a long time to catch on, but I can’t imagine liv without it today.