In argumentative writing, writers are expected to use hedged expressions and stance devices through specific linguistic expressions to convince their proposition effectively. Yet little research attention has been paid to whether the inclusion of such devices is related to the overall quality of second or foreign language learners' argumentative writing. In this study, hedges and stance devices that are included in 28 advanced Korean EFL writers' argumentative writing were analyzed to identify their potential relation to the overall writing quality. Analyses demonstrated that although hedges and stance devices were related to argumentative writing quality in general, the specific linguistic forms that predicted two different aspects of writing quality -- formal and content quality -- were different. Specifically, hedges played a significant positive role in only content quality of writing, and the specific stance devices that significantly predicted formal quality did not contribute to the content quality, and vice versa. The findings from this study provides important pedagogical implications for EFL writing instruction.