It's not either or.
The Soviets fought the Germans longer, and suffered the most casualties against them, but they would not have been able to make those stands without the monetary, food, and military equipment support the US provided. Moreover, the Anglo-American alliance drew a significant portion of the Nazi's best aircraft, tanks, units and leaders. Remember that Rommel was in the desert with Tiger tanks and a large German force when Stalingrad was liberated? Remember that the Nazis scoured the Eastern front bare in order to launch Operation Citadel, known as the "Battle of the Bulge?" Remember that the Nazi effort in the air was almost entirely consumed with defending Germany against Anglo-American bombers from 1944 on?
It's not an either or situation.