Communism is, conceivably, the perfect form of government. Every view that anyone has had of some sort of heaven or utopia has carried more similarities to communism than any other form of government. However, for communism to work there must be something in place to make people attempt to follow the rules. In the case of monasteries and communes, there is a deity that the people are attempting to follow. In Soviet Russia there was an army that kept everyone in check, and did brutal things to keep communism on top. Everyone sharing their belongings and work sounds good, but there is no way that man can possibly acheive it on their own. The main difference between capitalism and communism is that capitalism is ever changing and evolving with the society, where as communism stays unchanging. This is why America has held power for so much longer than any other country in recent memory.
However, people are correct in saying that American government is not purely capitalist. Although pure capitalism has never really been attempted, i.e. laissez faire capitalism, it is not hard to concieve that if there was not a government keeping it in check than capitalism would quickly turn to anarchy and chaos, just like communism. The only truly fair comparison would have to be between socialism, a less intense form of communism, and American style capitalism. I am more inclined to still lean towards American style capitalism, since there are endless examples of American style capitalism lasting longer than socialism.