While design principles for effective teacher professional development (PD) courses have been empirically identified in comprehensive studies, they have not yet been well disentangled with respect to the contribution of specific design elements. This particularly applies for digital online PD courses and for teachers with different prior knowledge. In this paper, we investigate two design elements for the systematizing phase (videos vs. discussions) of a synchronous online PD session with regard to their differential effectiveness for teachers’ professional growth in pedagogical content knowledge (PCK) and their emotional and motivational reactions to the PD. In our focused randomized controlled trial, 102 mathematics teachers (with heterogeneous PCK) participated in 2‑hour online PD sessions on specifying, noticing, and enhancing students’ understanding of multiplication with various PD inquiry activities. In the systematization phase, teachers were randomly assigned to two treatment conditions: the video group watched a systematizing video (i.e., a video that summarizes the PCK elaborated during the session) and the discussion group had small-group discussions about individual summaries. In both groups, teachers’ reports about emotional and motivational reactions to the PD revealed similarly high values, and their PCK improved significantly from pre-test to post-test. Analyses of variance with repeated measures revealed significant differences between the treatment conditions for teachers with low prior PCK: Teachers with low prior PCK who watched the systematizing video developed their PCK significantly more than those who discussed in small groups. In this way, the study contributes to disentangling the differential effects of design elements in online PD courses for teachers with heterogeneous prior knowledge.