Collaborative argumentation is a powerful means to promote conceptual understanding of scientific concepts for which students have robust misconceptions. However, eliciting and maintaining productive collaborative argumentation is known to be difficult. Moreover, it is unclear whether and how the factor of communication media might enable or inhibit argumentative discourse that is powered by collaborative argumentation, in particular for a certain pattern of argumentative discourse pertaining to productive outcomes. In the present work, using a controlled experiment, we examined the role of communication media in argumentative discourse during collaborative argumentation when fully considering pre-service science teachers as important participants for argumentation research. Two classes of 47 pre-service science teachers were allocated to one of two conditions (class A, 6 groups of 23 individuals: computer-mediated online context; class B, 6 groups of 24 individuals: face-to-face context), and participated in the collaborative argumentation activity. Quantitative results revealed pre-service science teachers in the both contexts exhibited a "U-shaped" pattern of argumentative discourse during collaborative argumentation--two high and one low: more deliberative argumentation than disputative argumentation, and more co-consensual construction than disputative argumentation. Results also indicated a difference: pre-service science teachers in the computer-mediated online context exhibited higher percentage of deliberative argumentation while those in the face-to-face context exhibited higher percentage of disputative argumentation. Through coding and analyzing participants' responses to stimulated recall group interviews, qualitative results further revealed pre-service science teachers' perceptions of conflicts during collaborative argumentation in the two contexts differed in several aspects: expressions of opposing ideas, acceptance of opposing ideas, exchanges between people who held opposing ideas, and perceptions of time. The current findings shed light on the role of communication media in argumentative discourse, deepening current understanding of how people perceive conflicts during collaborative argumentation in the computer-mediated online context and in the face-to-face context. Implications for science education practices were also discussed.