Both the Watergate scandal and the Iran-Contra Affair involved US presidents. In the Watergate Scandal, Richard Nixon and members of his administration were accused of taking part in and covering up a series of ilegal activities, such as breaking into Democratic Party buildings, spying political opponents and recording them. On the other hand, the Iran-Contra Affair happened during Reagan's Administration and it consisted in the illegal sale of weapons to Iran and the use of the money obtained to finance dissident armed groups against Nicaragua's Government.
One of the main differences between both events is that, while the Watergate Scandal revolved around domestic politics, the Iran-Contra Affair was related more to foreign policy. Furthermore, according to official investigations, Richard Nixon was more involved in the Watergate scandal than Reagan was in the Iran-Contra Affair: if Nixon had not resigned, he would have been impeached by the Congress and the Senate, while Reagan was believed to not have known fully the extent of the operations regarding the sales of weapons and the financing of the Nicaraguan opossition.
The Watergate Scandal seems to have affected more the US than the Iran-Contras Affair: the political crisis unleashed by the scandal made president Nixon resign, while Reagan was left in the office.