Upgrading to openHAB 2.5 by updating existing 2.4 or clean install 2.5 with restoring configuration?

IMHO: if you have the time to go with the second option - go with the second option. And use the time to write down for yourself what extra steps you take for a new installation (example: https://community.openhab.org/t/openhabianpi-initial-setup/).