This study examined the preferences for professional development (PD) in primary (elementary) science among pre-service teachers (PSTs) and in-service teachers (ISTs). The contribution of the study is its focus on quantifying the relative importance of factors that were significant for teachers by using best-worst scaling methodology. Rather than considering potential factors in isolation, teachers traded off among content areas of PD, thereby revealing which aspects they most preferred. A comparison of PSTs and ISTs indicated that both sought greater guidance on adapting their science teaching for multi-age classes and on strategies to engage students in activity-based science. Relative to the PSTs, the ISTs reported less need for PD opportunities that emphasized collaboration and networks, and science pedagogy and content. Both groups indicated that they would most benefit from PD that focused on building their knowledge and strategies for teaching guided inquiry and investigation- and activity-based science. The findings offer critical insights into the broader improvement of PD of teachers in the context of science education.