25th Amendment differs from impeachment, since there's an explicit process to return to office with the former, whereas with the latter you can be barred from ever holding public office again.
Also, Sec. 4 requires that it be the Vice President and a majority of the Cabinet that ask Congress to remove the President... which is a bit of an issue, since the President can fire the Cabinet at will (see, e.g., Myers v. United States, though in dicta).