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They both believed that there was a superior race and it was their race
Well, not so much about race, but it might be more accurate to say that they were both dictatorships.
Stalin was a die-hard Communist -- it's true that he pushed around a number of minority groups in the USSR, but above it all, he was a flag waving, Communist cheerleader who didn't hesitate to take out anyone, whatever race or creed they might be, to build up the Soviet Union and his own power.
Hitler was convinced that Nazism was the way forward, but he explicitly added a racial agenda to his ideology which expressed itself in the Final Solution. Stalin didn't need a Final Solution -- anyone was could be a target for his secret police and the gulags.
They were both totalitarian systems. One administrative party was in charge, lead by a dominant figure. The government had no bounds. (Examples include extermination systems for both of them, concentration camps by the Nazis, and gulags by the Soviets).