There will be much added to this particular comment on the question. I think that this might be the start of something more intricate, something in which further study will prove to be a good thing. On one hand, I think that the role of the President in both was to act in a manner outside of Congressional understanding, approval, and consent. In Watergate, the President was operating outside of Congressional oversight. President Nixon used "Executive Privilege" to mean much more than it was originally intended in terms of seeking to consolidate his own power. The actions of Nixon as President in the commissioning of the Watergate burglary as well as in his actions, in general, as President regarding his attempts at reelection for the basis of the Watergate Scandal and reflect how the President ends up becoming reprimanded by the other two branches of government when the boundaries of his office are eviscerated for personal gain.
The Iran- Contra Affair took the issue of Presidential negotiation in foreign treaties and arrangements to a different and disturbing level. Members of the Executive Branch were selling arms to Iran for profit and release of American hostages in Beirut and then diverting profits to fund the overthrow the Sandinista government in Nicaragua. In this, there is much in way of the President operating outside of Congressional oversight. The Congress was never notified of these actions, and it could be argued that President Regan and his aides operated in opposition to Congressional legislation that stated that federal monies could not be used to intervene against the Sandinista government. In this, the President operated in an international realm outside the domain of Congressional oversight. In the end, the President was not proven to be as legally guilty as in Watergate, but many of the same elements of conspiracy, cover-ups, and lack of transparency is evident in the relationship between the President and the other two branches of the government.