I'd like to ask if there's a difference between:
Will there be a huge difference? In terms of disk space and speed of the machine after installing the Catalina?
I feel like the #2 (my usual way) is more practical in terms of time consumption and effort.
A "Fresh install" usually means erasing the target volume of all files, and then installing an OS, thus removing any possible problems from third-party installed software, and incorrect or inappropriate settings in /Library or /User/Library .plist files.
Simply deleting all the files in one or more user accounts will not remove third-party applications, their /Library files, other installed processes, and things like printer drivers.
So, the latter will keep more data on your drive than a fresh install. Speed is unlikely to be affected, unless your drive was full.
I can't think of a reason why deleting user accounts would be a useful process.
By default, Apple's system installation process upgrades the existing OS, and retains all user files, settings, applications, etc in place. Some listed items, known to be incompatible, are removed by the installer to folders where they are not launched by the OS.
As Apple releases a new OS every year (or every 2 months if you count the point updates): 'slashing and burning' all your settings, files, and applications is entirely unnecessary. (I have updated in place every OS from 10.2 Jaguar to 10.15 Catalina, and cloned my disks across more than six Macs. I dare say there's some 'cruft' from my 2002 G3 iBook now sitting on my 2018 Mini!)
I would recommend only doing a clean install, or other similar process, if you get some problem occurring after upgrading. Alternatively, with a bit of detective work, you can usually find out the 1% of your files that are causing the problem, and then delete those, rather than the removing 100% and putting 99% back.
