A simple upgrade is the easiest way of installing macOS Mojave (due today). It saves all your personal files, apps and user data, while a “clean install” will erase all data on the startup drive and replace it with a clean copy of macOS Mojave.
If you’ve noticed that your Mac is running slower, has startup or shutdown issues, or frequently crashes, a clean install could be a smart move. Here’s how:
Make a full backup (or, even better, two) of your data from disk. You can manually copy files to an external disk. Even better: use Time Machine or Carbon Copy Cloner to make an exact clone of your Mac’s disk.
Next create create a Mojave installer on a USB drive (16GB or bigger and without any data on it).
Connect the USB drive with the version of macOS Mojave to your Mac.
Restart your Mac while pressing and holding the Option key.
The Startup Manager will launch. Select Disk Utility and erase your hard drive.
Return to the main menu and choose Install.
Follow the instructions to launch macOS Mojave from your USB drive.
Some of the info from this article came from the Italian website, Melarumors.