Either one is perfectly ok if you want to start anew. After erasing the startup disk and installing the macOS version you want, the installer will invoke what Apple used to call Setup Assistant which is literally the same thing as Migration Assistant but performed prior to installing any User Accounts. What that means is when asked if you want to transfer information, select the Time Machine backup.
That procedure is the default in Big Sur. Read all about it here: Restore your Mac from a backup - Apple Support.
The subtitle provides a subtle hint:
"Use Migration Assistant to transfer files from your Time Machine backup to your Mac."
and if you didn't get the hint:
"If you need to reinstall macOS, do that before continuing."
Disregard my opinion if you want, but never once in the history of macOS or its predecessors have I ever found it beneficial or necessary to perform the so-called "clean install". There are more problems reported on this site resulting from that than simply upgrading the way Apple designs upgrades to be. Every Mac I have has been upgraded (or migrated) from Macs going back to the original one in 1984. It works.
