Why Nixon would have been impeach if he didn't resign
Article I: Obstruction of justice: in Nixon’s case, included efforts to delay, cover up, or conceal evidence relating to the Watergate break-ins by making false and misleading statements, withholding information, encouraging witnesses to give false or misleading statements, attempting to interfere with the FBI and other investigators, leaking information about the investigation to help the accused, and insinuating that people who refused to testify against him or who gave false testimony would gain favors.
Article II: Abuses of presidential power: misusing the FBI, Secret Service, and other government employees by allowing their information to be used for purposes other than national security or the enforcement of laws; using campaign contributions and the CIA in an attempt to sway the process; misusing executive power by interfering with agencies within the executive branch.
Article III: Defiance of subpoenas: involved Nixon’s willful disregard of subpoenas of, and failure to produce, papers and information for the House Judiciary Committee
Why Clinton was impeached:
Article III: Obstruction of justice: determined that Clinton had prevented, obstructed, and impeded the administration of justice, and that he had engaged personally, and through his subordinates and agents, in schemes designed to delay, impede, cover up, and conceal the existence of evidence and testimony related to a federal proceeding.
Trump exceeds all these criterion. I think Nixon and Clinton were ethically heads and shoulders over Trump.