From a technical perspective a clean install is always best. It eliminate carry over issues, as well as potential glitches from the previous OS. At the same time you have the chance to fully clean your machine of any unwanted stuff (malware, 3ed party apps, plugins) that you don’t want on your machine, or didn’t realize were installed. However it’s the longer route, as you’ll need to backup data (I would recommend doing this in either case), install windows, and then reinstall all the applications you need. The upgrade goes on top of the current OS. It is faster and easier from a non-technical user perspective. You don’t have to reinstall any application, so long as they are compatible.