Timothy Snyder's book Bloodlands studies how the interaction of these two systems led them both to become more violent.  
Two answers requiredA 2008 book edited by Michael Geyer and Sheila Fitzpatrick attempted to make comparisons between these two political systems. In a certain "Black Book," Stephane Courtois argued that one of these systems borrowed methods of violence from the other, and attempts to tally the casualties of the latter system.  Timothy Snyder's book Bloodlands studies how the interaction of these two systems led them both to become more violent.  The "Historians' Quarrel" was a debate between historians who believed that these two systems were comparable and those adhering to the (*) Sonderweg thesis, which argued that one of them was incomparably evil.  The most famous study of these two systems argues that they attempted to destroy class divisions in order to create "masses", and relied on "ideology and terror" to exert control over their populations.  For 10 points, identify these two systems, which were studied by Hannah Arendt in The Origins of Totalitarianism and which clashed during World War II.
Two answers requiredA 2008 book edited by Michael Geyer and Sheila Fitzpatrick attempted to make comparisons between these two political systems.
Nazism and Stalinism (accept in either order; accept equivalent answers such as "Fascism and Communism" or the "Nazi system and the Soviet system")