Expanded from an earlier answer of mine

This sounds like a bad or superficial test, because there were major differences between the ideology of Stalinism and National Socialism.

The idea that Stalin and Hitler represented two sides of the same authoritarian coin was one that dated back to when the actual dictatorships existed. The German Marxist emigre Otto Rühle would maintain in works like "The Struggle Against Fascism Begins with the Struggle Against Bolshevism" that Stalin's state was simply "Red Fascism" while Hannah Arendt would argue that both dictatorships exerted power in near-identical ways by fragmenting and terrorizing populations. Much of this critique actually picked up on the fascist concept of a totalitarian order in which the state organized and regimented society. While some of Mussolini's ideologues use totalitarianism in a positive sense, the term acquired a pejorative sense by the 1940s. The totalitarian theory emerged as one of the dominant paradigms in US academia during the first half of the Cold War and it enjoyed a wide currency among intellectuals. Counter-intuitively, totalitarianism held sway among a broad political spectrum, ranging from far-right anticommunists like the John Birchers, to centrist Cold War liberals, and disaffected Trotskyites and others on the Left like the Schachtmanites who felt that the Soviet experiment went off the rails into dictatorship because of the ideological deficiencies of the helmsmen.

The problem with totalitarianism is these comparisons only work on the most superficial levels. Both regimes had leadership cults, single party systems, secret police, and other benchmarks of the modern dictatorship for sure, but how each of these served as a mechanism or authoritarian rule varied quite widely. The NSDAP for example, sought to coordinate and bring in a broad swath of German society and co-opt existing structures. The Gestapo and other security organs recruited among various existing German police and disaffected professionals (Wildt's "uncompromising generation" of radicalized young men) while the Cheka/NKVD created its own police organization more or less from scratch. The trajectory of the CPSU under Stalin operated quite differently with it creating a number of structures rather than coexisting with existing ones. Stalin's leadership cult was a different matter than that of Hitler's. A cult of personality in an avowedly Marxist-Leninist state was itself a contradiction; Marxism was about the evolution of broad social forces, not the individual leader transcending them. Stalin instead preferred his depictions to not be Stalin the man, but rather Stalin as the symbol of the cause. In contrast, the Hitler cult was deeply personalist and wrapped up Hitler's own self-inflated biography as a genius with the a political philosophy of Führerprinzip. Hitler himself was an load-bearing member of the National Socialist state as there was very little thought given to succession and none of his entourage had close to a mass following to continue the cause. The Soviet dictatorship in contrast not only survived Stalin's death but reverted to the type of collective leadership with a "first among equals" like Khrushchev that had characterized the Lenin period.

Racism and other forms of ethnic-based discrimination were another departure between the two ideologies. While there was a low-grade antisemitism within the Soviet system as well as an increasingly patronizing approach to non-European minorities within the USSR, biological racism did not have much of an official sanction within the USSR. Quite the opposite, prior to 1941, open outbursts of antisemitism were a disciplinary offense and the Stalinist state associated racism as a tool of fascism. Stalinist repression of Jews may have been encouraged by lingering antisemitism among the state authorities, but there needed to be some other rationale to officially justify it. The postwar antisemitic turn like the Doctor's Plot was justified not because Jews were innately traitorous, but because the state alleged a Zionist plot was forming within the USSR to undermine the state. This is quite different than the Third Reich which placed biological racism at the center of many of its various policies. The purging of Jews from German life was not because of the actions of a few, but because of who the Jews were in the Nazi racial schema. In the USSR, a Sovietized Jew (that is non-practicing and holding the "right" kind of politics) could achieve a degree success within the system, most famously in the example of Molotov, even if antisemitism made Jewish background a dangerous liability. The idea of a Nazified Jew was antithetical to the whole National Socialist Staatsidee. Even those individuals with partial Jewish heritage like Erhard Milch had to both demonstrate their indispensability as well as have the right social networks to avoid summary dismissal and discrimination.

Nationalism also shows another point of divergence. It is not too controversial of a point to claim that the Third Reich was a hypernationalist state and Third Reich ideologues often claimed all of the German heritage as precursors to the movement. The Stalinist approach to Russian history and nationalism was much more selective and only gathered steam during the Second World War. Only a few tsarist figures were reappropriated as Bolsheviks born in the wrong era; typically these were military generals, intellectuals, and artists. And although the Great Russian nation became the first among brothers within the Soviet family of nations, the Soviet state did not try to erase recognized nations either. The policy of korenizatsiia (indiginzation- termed by Terry Martin Soviet affirmative action) allowed Soviet national minorities a degree of power within their own SSRs. Granted such power was mediated through the Communist Party and the local state bureaucracy and it did presuppose a degree of supplication to Soviet power, but it was still a mechanism for non-Russian participation in the Soviet experiment. The Nazi approach to national minorities had no such structures aside from extemporized categories of racial lieutenants like the Slovaks. Additionally, while Stalin's ethnic cleansings such as that of the Crimean Tartars could be quite severe, the USSR never made the conceptual leap that these ethnic groups did not deserve a right to exist. Nazi thinking of defined racial enemies was genocidal in its impulses from the very start and the right conditions made an already violent form of discrimination truly murderous.

Other areas also show broad differences between the two states. Soviet developmentalism and industrialization under Stalin reflected various precepts present in Marxist-Leninism with an aim to a certain end, namely the erection of socialism and the creation of a communist utopia. Massive industrial projects like Magnitogorsk were a Promethean attempt to bring about an industrial transformation of the country. The mass projects of Hitler served to glorify the existing order, even if they had an ideological gloss of being in the service of a "thousand year Reich." Moreover, the reams of social history produced on Germany since the 1960s and the USSR since the late 1980s has shown that the lived experience of both dictatorships was markedly different.

Comparison between Nazi Germany and the USSR under Stalin really don't illuminate much to understand either regime. The broad comparisons are there, but the details are quite important. A Ferrari Testarossa may share a number of features in common with a Ford Taurus- both were means of automobile transport introduced in the mid-1980s, have four rubber wheels, steering wheels, brakes, bumpers, use bovine branding, can be initialed as F.T., etc.- but one car is clearly different than the other.