An important skill for L1 language teachers when teaching grammar is the ability to produce and quickly evaluate arguments that underpin a grammatical analysis. Previous research has revealed that the strongest arguments in favour of a particular grammatical analysis are based on linguistic manipulations (LM) rather than on rules of thumb (RoT). This makes it critical for teachers to be able to handle arguments based on LM. If LM are considered too difficult to process compared to RoT, however, (student) teachers may avoid grammatical argumentation based on LM altogether, and they might struggle to evaluate their pupils' LM-based grammatical argumentation. The current study has therefore examined whether LM impose a higher cognitive load on Dutch student teachers than RoT, using grammatical discussion tasks in which participants (N = 298) evaluated arguments based on RoT and on LM. Multilevel analyses indicate that LM are indeed more difficult to process than RoT, as measured by response times, correct classifications, and perceived difficulty ratings. This effect is partly influenced by student teachers' need for cognition and their willingness to engage in grammar.