The difference is -- Nazis were much more brutal. Genocide was integral part of Nazi ideology and practice.
US Ambassador to the USSR in 1937-39 Joseph Davis on communism and Nazism and their fundamental difference (July 7, 1941)
My friend Lindbergh surprised me by saying that he preferred Nazism to communism. In general, making such a choice is a desperate thing, but the difference between these two subjects is too great. Both Germany and Russia are totalitarian states. Both are realistic. Both of them apply strict and ruthless methods. However, there is one significant difference that can be shown as follows. If Marx, Lenin, or Stalin were Christian believers, and if one tried to place the communist experiment carried out in Russia within the framework of the dogmas of the Catholic or Protestant Church, the result would be declared the greatest achievement of Christianity in the entire history of mankind in its quest for humanity and the implementation of Christian commandments in the life of society. The fact is that the Christian religion can be combined with communist principles without committing much violence to its economic and political goals, the main of which is the "brotherhood of all people". If we conduct a similar test in relation to Nazism, we will find that it is impossible to combine the two ideologies. The principle of Christian ideology cannot be imposed on the Nazi philosophy without destroying the political basis of the state. Fascist philosophy creates a state that is actually based on the denial of the altruistic principles of Christianity. For the Nazis, love, charity, justice, and Christian values are only manifestations of weakness and decline if they contradict the needs of the state.
This is the whole difference - the communist Soviet state can act with Christianity as the basis for achieving the ultimate goal - the universal brotherhood of people. The Communists allow the state to die out as the individual improves, while the Nazi ideal is just the opposite - the state is above all else."
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By the way, about brutality of the USSR, would you believe that fewer people were incarcerated during Stalin's time than are now in the United States?