Both presidents' policies were largely dictated by necessity. President Roosevelt needed to establish an alliance with the USSR to help win the war. Once the war had been won, and with Russian boots on the ground in Eastern Europe, Roosevelt had little choice but to accept the harsh realities of the new political situation. Although critics railed against what they perceived as a sell-out at Yalta, in reality there was very little else that Roosevelt could have done under the circumstances. The Soviet Union was an atomic power, and as such direct armed confrontation between the two superpowers was unthinkable.
Truman understood this, which is why he adopted the strategy of containment to deal with the growing Soviet threat. Soviet domination of Eastern Europe couldn't be challenged directly, but the spread of Communism could nonetheless be contained—hence the implementation of practical steps by the Truman administration, such as the Marshall Plan. Truman's Soviet policy was perhaps bolder and more imaginative than Roosevelt's, but that was mainly because it had to be.