In peer assessment, both receiving feedback and giving feedback (reviewing peers’ products) have been found to be beneficial for learning. However, the different ways to give feedback and their influence on learning have not been studied enough. This experimental study compared giving feedback by writing comments and by grading, to determine which contributes more to the feedback providers’ learning. Secondary school students from Russia (n = 51) and the Netherlands (n = 42) gave feedback on concept maps during a physics lesson. The lesson was given in an online inquiry learning environment that included an online lab. Students gave feedback in a special Peer Assessment tool, which also provided assessment criteria. Findings indicate that post-test knowledge scores were higher for students from the commenting group. The difference between the groups was largest for the low prior knowledge students. Possible educational implications and directions for further research are discussed.