Reasoning is the development of a logical chain of deductive thought, from A to B to C. Argumentation is generally taken more broadly as the act or process of giving reasons for or against something. Thus reasoning is seen as a logical act, while argumentation includes eristic dialog, which is that branch of social debate in which victory over an opponent, rather than the development of a logical chain of thought, is the primary goal.
Is your research, then, to understand the social epistemology of students ("I believe x, because I have heard it from my peers, and all of them seem to believe it; therefore it must be so")?