The Watergate scandal involved the break-in by the Republicans into the Democratic National Convention headquarters at the Watergate Hotel in Washington, DC in 1972. This criminal activity became a constitutional crisis because the Nixon administration attempted to cover-up the event. In the course of investigating the break-in, numerous abuses were uncovered by journalists (Bob Woodward and Carl Bernstein at the Washington Post)and investigators. The arrest of the five burglars and subsequent investigations by the FBI found a link between the burglars and the Pre-Election Campaign Fund for Richard Nixon. An investigation of former Nixon staff, revealed an elaborate tape recording system in the Oval Office. Once the courts ordered the release of the tapes, it was clear they implicated Nixon in the break-in. Rather than face impeachment, Nixon resigned before Articles of Impeachment could be voted on in the U.S. House of Representatives. This scandal tarnished the presidency and exposed fundamental flaws in Nixon's character and decision making abilities.
Unlike Watergate, the Iran-Contra Scandal involved the selling of arms to Iran and the proceeds from those sales being funneled to the Contras in Nicaragua who were fighting the Sandinistas. These activities were spearheaded by members of the Reagan administration during his second administration (1984-1988). Initially, the administration argued the funds were being used to free American hostages held in Lebanon by Hezbollah, an Islamic terrorist group. In 1985, Colonel Oliver North was implicated in diverting money to the to the Contras. The scandal was investigated by the U.S. Congress and the Tower Commission, a three person appointed by Reagan. The investigation never implicated Ronald Reagan.
The Watergate scandal was clearly the more damaging. It weakened Americans faith in the presidency and led to the resignation of Richard Nixon.