Land use change, natural resource use and climate change are challenging Sustainable Development issues (SDGs 13–15). Fostering the competencies to deal with such issues is one core task for current educational endeavors. Among these competencies, decision-making competencies are central. In detail, we investigate how learners evaluate alternative decision-making options to improve existing competence models. We exemplify our competence modelling approach using the designation of a Marine Protected Area. The cross-sectional sample consists of secondary school students and student teachers (N = 760). Partial Credit modelling shows that quantitative modelling of decision-making options is a different competence dimension than perspective taking if contextualized for Sustainable Development. In quantitative modelling, mathematical modelling is used to evaluate and reflect on decision-making options. Perspective taking covers the ability to consider different normative perspectives on Sustainable Development issues. Both dimensions show plausible (latent) correlations with related constructs within the nomological net, i.e., with qualitative arguing, economic literacy, mathematical competencies, reading competencies and analytical problem solving. Furthermore, person-abilities increase with level of education for both dimensions. The identified competence dimensions quantitative modelling and perspective taking were successfully modelled and shown to be distinct; the resulting measuring instrument is reliable and valid.