"The decision on Tuesday to bar the presidential candidacy of Ali Akbar Hashemi Rafsanjani, a founding father of the revolution and a former president, shocked Iranians, particularly among the 70 percent of the population that is under 35 and grew up when he served in many leading positions." - Thomas Erdbrink, "A Founder of the Revolution Is Barred From Office, Shocking Iranians," The New York Times, May 21, 2013.America is an oligarchy; Iran is a theocracy.
Actually, it isn't that simple. Iran's system of government has oligarchical elements, and America's leadership is more theological than most people like to think.
The elections in both countries are also very similar. The Guardian Council in Iran approves a list of candidates, but at least it does it publicly so people know who is really in charge. America's equivalent of a Guardian Council is in the shadows, and it operates through transnational oligarchical organizations like the Bilderberg, making it more difficult for well-meaning voters to look behind the curtain and realize that the people's voice doesn't matter at all.
So, which system is better? Neither. America and Iran are both tyrannies wherein the people's opinions are not considered by the elites. The only difference is America is an aggressor and it spreads its tyranny through war and economic means.