Before anything else, I think you should consider and keep in mind that Truman's March 12, 1947 speech (which is understood as forming the basis of the Truman Doctrine) had a very different intent, and came out of a very different political context, than Kennedy's inaugural address. Inaugural addresses serve as a political thesis statement by which the incoming administration presents its fundamental vision of what it aims to achieve, and Kennedy's speech adheres to that same basic goal. Truman's speech, on the other hand, was inspired by a very concrete set of political events unfolding in Greece (as well as Turkey), and its audience was not so much the American people or the world (as Kennedy's was), but rather the US Congress. Its primary objective was to convince Congress to authorize financial aid to the countries in question.
A discussion on the similarities and differences between these two speeches can address a variety of different points and subjects. For example, while both speeches introduced large-scale policy proposals and invoked a vision of the United States' role in the world, at the same time, their respective visions had radical differences. I'd suggest you think carefully on Kennedy's words towards the communists, for example, as well as his thoughts concerning nuclear weapons, and then compare this message with the one you'd find in Truman. Both speeches placed a focus on the ideal of freedom, but does the immediate context of World War II reflect itself in Truman's speech in ways that it does not in Kennedy's? Even if both speeches would give the United States a critical role in protecting freedom, do they give it the same role in doing so, and the same place in the international community? In terms of content, and the visions that these speeches espouse, there's a lot worth writing and thinking about.