The present study explores the independent and interactive effects of task complexity and task modality on linguistic dimensions of second language (L2) performance and investigates how these effects are modulated by individual differences in working memory capacity. Thirty-two intermediate learners of L2 Spanish completed less and more complex versions of the same type of argumentative task in the speaking and writing modalities. Perceived complexity questionnaires were administered as measures of cognitive load to both L2 learners and native speakers to independently validate task complexity manipulations. Task performance was analyzed in terms of general (complexity and accuracy) as well as task-relevant (conjunctions) linguistic measures. Quantitative analyses revealed that task modality played a larger role than task complexity in inducing improved linguistic performance during task-based work: Speaking tasks brought about more syntactically complex output while writing tasks favored more lexically complex and more accurate language. In addition, relationships of working memory capacity with various linguistic measures were attested, but only when the cognitive complexity of tasks was enhanced.