REMEMBER: with this type of question it is not so much WHAT you say but HOW you say it.

Therefore you don't have to write exactly what is suggested here -
you can argue your own case.
However, try and adopt this style.

Which was more important as a reason for the rivalry between the USA and the USSR 1945-1949?
Soviet expansion into East Europe and Central Europe
The Truman Doctrine and the Marshall Plan

Both the Soviet expansion into Eastern Europe, and the Truman Doctrine and the Marshall Plan were significant factors in the rivalry between the USA and USSR, but I believe that the Soviet expansion was more important primarily because it caused the USA to introduce the Truman Doctrine and the Marshall Plan.

Soviet expansion into Europe was more significant, as it consisted of many acts of aggression. After Yalta and Potsdam, it had been agreed that Eastern Europe should be a Soviet sphere of influence, but also that these countries should have free elections. However, due to the state of economic depression many European countries were experiencing, many began to resort to Communism. Stalin, actively or passively, helping this transaction, and in February 1948, Czechoslovakia had become Communist, which was particularly important as it meant that the ‘Iron Curtain’ was complete, and Stalin had created a barrier of satellite states between him and the west. However, Soviet expansion wasn’t incredibly significant as it had been agreed that Eastern Europe should be a Soviet sphere of influence, so Stalin wasn’t doing anything unforeseen, and in most cases, never invaded e.g. Hungary in 1947, the Communists became the largest party and began to imprison opposition leaders.

The Truman Doctrine and Marshall Plan, on the other hand, were simply the American response to Soviet expansion, and an attempt to contain Communism. They were non-aggressive tactics and involved trying to help countries recover economically as opposed to making them abandon their entire economic structure as did Stalin’s expansion, so they were less significant. In total $17 billion was given in aid to European countries which helped stop the spread of Communism, but Stalin saw it as a blatant attempt to gain in Europe and even interfere with his sphere of influence. This meant that he became very suspicious of the West, which in turn led to the first potential flashpoint of the Cold War – the Berlin Blockade.  Also they did make the economic divide between East and West more pronounced and led to the formation of the Communist COMECON.

Overall, both factors were extremely important reasons for the rivalry, but the Soviet expansion was more important as it involved acts of aggression, while the Truman Doctrine and Marshall Plan were simply economic attempts to counteract Stalin’s expansion.