Hello,
Everything has its advantages and disadvantages.
If you make clean install, of course the hard drive space is less occupied. But you have to make installation of all your programs and application again. Also, you need to activate the system again (if you're on retail license).
If you make an upgrade, you don't need to install all the programs again. There is no need of re-activating your system. However, files from previous version may remain on system which will still occupy hard drive space. Usually, they are moved to Windows.old folder. So if you remove them from the Windows.old folder, you can gain the hard drive space back.
Regards.