This study set out to examine which peer review, face-to-face given orally or online given in writing, is more effective in improving the overall argumentative writing achievement of English as a foreign language (EFL) university learners. The study utilized an experimental design and reported on one experiment including online peer review (OLPR) which was the experimental group (n = 74) and a face-to-face peer review (FTFPR) which served as the control group (n = 48). Both groups (n = 122) were trained in the use of their respective peer review, OLPR or FTFPR. Both groups wrote two argumentative synthesis essays, immediate and delayed, in two drafts. The second draft of each essay was written after receiving the respective peer review, OLPR or FTFPR. Two Multivariate Analysis of Covariance (MANCOVA) tests and a qualitative analysis of peer reviews were conducted to address the study question. Quantitative findings showed that participants in the OLPR group significantly outperformed their FTFPR counterparts in improving the argumentative synthesis writing of EFL university learners. The qualitative analysis of the peer review forms, and the revised essays revealed that the OLPR group gave more systematic feedback than that of FTFPR, OLPR focused on content, organization and language while commenting on the writing strengths and weaknesses. Thus, instructors are advised to use OLPR in argumentative writing classes. The study also showed the significance of shifting the control of feedback from the teacher to students.