Engaging students in scientific argumentation is a hallmark of high-quality science instruction. Despite the importance of this practice, it is not a ubiquitous one within K-12 classrooms since scientific argumentation is complex and difficult to learn how to do well. In this study, we examined similarities and differences in how preservice teachers (PSTs) and in-service teachers (ISTs) facilitate an argumentation-focused discussion with five upper elementary student avatars in an online, simulated classroom. Developing a better understanding of how science teachers at different stages of their career foster scientific argumentation will support teacher educators and researchers in designing more directed interventions to scaffold teacher learning of this complex practice. Findings showed that most teachers provided opportunities for students to engage in both argument construction and critique during these discussions. In addition, both PSTs and ISTs frequently prompted students to reference data or observations to engage in argument construction and were most likely to prompt students to agree or disagree and explain the reason for their agreement or disagreement to engage in argument critique. Finally, findings showed that ISTs used a wider variety of talk moves and used them more frequently in an integrated fashion throughout the discussions to engage the students in scientific argumentation. Implications for studying and promoting teacher learning of this complex teaching practice and for integrating simulated classrooms as practice-based tools in science teacher education and professional development settings are discussed.