I work in the software industry, darling, and, although I'm not in QA, I do a fair amount of testing of new releases (I write documentation). I end up testing both clean installs and upgrades, and in my experience, on most properly-working systems, there's not usually a huge advantage to the clean install, and having to reinstall your files, reorganize everything, and reset your user-specific configurations and such can be a serious pain.
My QA guys prefer clean installs when they're testing new and updated applications because the clean install ensures that the computer is free from any corrupted files, and because there's less chance of conflict with hidden and leftover junk. When you uninstall an application, very often the uninstall isn't completely clean, and there are files left behind (this is especially true with applications in development, which is what my guys test), and these files can sometimes cause odd behavior in a new application due to resource conflicts and so on. By wiping the machine before installation, my guys ensure that the behavior they see is really from our product.
If your computer is behaving itself now, I'd go with the upgrade, rather than the clean install...if you run into any problems, you can always re-run the install from scratch. If it's especially slow and cloggy and acts oddly, then I'd run the clean install to get rid of anything that could impact its behavior.
Of course, before you make any major changes, make sure that you have a full backup of all of your files. Maintaining current backups is one of the most important things you can do to protect your data...there are backup solutions that also help you to migrate applications to a new operating system if you choose to do a clean install. If you choose to do the upgrade, make sure you run a virus scan on your current configuration before you begin (I like to use two different products like malwarebytes and Windows Security Essentials - the different signatures can pick up viruses and malware with one product that the other product missed).
Hope that helped!
(((Ethan)))