The Platinum style is no longer available with Qt4, but is still in Qt3. The Mac style is in both Qt3 and Qt4 (though considerably improved in Qt4).

The differences between these styles would be huge. Platinum is uses QPainter drawing code, and is thus available on all platforms with Qt3 (including the current KDE under X11). The Mac style is for OSX only, as it uses native OSX drawing routines. It isn't Qt drawing the theme, but OSX itself. Because of this you can't compare the two styles.

Also, there are no "windows without any kind of style". Every Qt application has a style. The common base style is QCommonStyle, but it is an abstract class that cannot be instantiated directly. It is a good starting point to make your own styles with. The Platinum style inherits from the Windows style, which in turn inherits from QCommonStyle.