The Germans killed three million Jewish Poles and 1.5 million non-Jewish Poles. The Soviets killed 150,000.

To add to estherke's comment, Soviet occupation policy, while capable of very horrific abuses of human rights, was never genocidal in its intent. The Soviet authorities were able to rationalize away all sorts of actions in the name of national security or ideology, but clearing out whole populations was never a central plank in their agenda. Even though their occupation policy of population transfers during 1944-46 amounted to ethnic cleansing, the Soviet regime never made the conceptual leap that certain nationalities do not deserve to exist. The Third Reich was the opposite in that it conceived of most of Polish space to be fundamentally German and initiated a series of programs and initiatives to eliminate any Polish or Jewish presence.

If you're interested, check out Timothy Snyder's Bloodlands, which is a comparative history of this period and part of what Snyder does very well is demonstrate how both the USSR and Nazi Germany defined themselves against each other's actions.