Previous work on non-native speech rhythm has often drawn on L2 phonological theory for the interpretation of findings. The explicit confrontation of theory-derived hypotheses with data remains scarce, however. This paper illustrates how a hypothetico-deductive approach can contribute to our understanding of L2 speech rhythm. We consider cross-sectional data on prominence alternations in German learner speech from the viewpoint of two dynamic frameworks: The Ontogeny Phylogeny Model (OPM) and the Linguistic Theory of L2 Phonological Development (LTD). While both theories deal with L1-independent, universal forces in L2 acquisition, the OPM further considers the role of L1 transfer, similarity, and markedness. The predictions we formulate based on the two models lead us to pursue distinct methodological strategies. While our reading of the OPM prompts us to measure speech rhythm as a single, global category of speech, the LTD suggests a more nuanced, componential approach to L2 rhythm. Our application of the OPM confronts us squarely with the limited utility of rhythm metrics for L2 speech research and points to a number of issues at the theory-data interface. Overall, the LTD generates more informative predictions and provides a richer framework for the empirical study of prominence grading in L2 speech.