Oil-rich nation: this description matches both countries. Iran and Iraq are very rich in oil sources.
Predominantly Sunni population: Iraq. This Gulf country has a vast Sunni majority, although there is a large percentage of Iraqi citizen of Shia tradition.
Predominantly Shia population: Iran. Shia Muslims are around 10% of Muslims in the world and Iran has the largest Shia population in the world. In fact, this country presents itself as a leading authority for Shia believers.
Powerful leader was Ayatollah Khomeini: Iran. This political figure was the leader of the Iranian Revolution of 1979 that overthrown the Shah's regime and established the Islamic Republic of Iran.
Powerful leader was Saddam Hussein: Iraq. Saddam Hussein was a powerful Iraqi dictator who remained in power from 1979 until 2003. He started the Iran-Iraq war during the 1980s, invaded Kuwait in 1990, and faced the First and Second Gulf Wars. He was overthrown by the US invasion of Iraq in 2003.