Mac is more reliable, Windows is cheaper. Actually, if you only want to do the kind of stuff you listed you might be best-served with a Chromebook. They’re cheap, lightweight and much less vulnerable to viruses than either (if you need to turn in a .docx you can download Google Docs as docx).
Most modern computers should have no difficulty lasting for four years; it’s just a matter of how responsible you are using it (which especially applies to Windows, and a little bit to Mac as well).