In this study we looked into the learning affordances of task modalities as mediated by task complexity. A group of intermediate learners performed an argumentative task with two levels of complexity orally and in writing and their performance was rated for adequacy, and assessed in terms of the CAF (complexity, accuracy, and fluency) measures. In both oral and written modes, communicative success was found to be linked to lexical complexity and fluency. However, adequacy was associated with accuracy only in speech and with propositional complexity (idea units) only in writing. Task complexity did not moderate the links between communicative adequacy and the CAF dimensions. Based on these findings, implications for task design and for language-learning potential across modalities are drawn.