As we have discussed, many IDE functions are shared between PythonWin and IDLE (or more correctly, stolen from IDLE by PythonWin, with Guido’s blessing). So the differences between PythonWin and IDLE have less to do with features and more to do with platforms.
IDLE runs on almost all platforms that support Python. Although the user interface it provides isn’t quite as slick as PythonWin’s, you do have an IDE that runs places other than Windows. For many people, this is the single killer feature!
Of course, your choice of environment is up to you, but it appears that people who use Python on multiple platforms prefer IDLE because their environment can follow them. People who use Python only on Windows typically prefer PythonWin and its interface. As IDLE evolves further and the Tkinter GUI framework offers more features on Windows, we may find this simple distinction blurring. We recommend you spend time with both IDE environments and choose for yourself.