Argument writing is challenging for elementary students. Previous experimental research has focused on scaffolding rhetorical goals, leaving content goals relatively unexplored. In a randomized experiment, 73 students in grades 5, 6, and 7 wrote persuasive texts about difficult-to-classify vertebrates. Each student received one of three sets of writing prompts: a persuasive goal only (control); a persuasive goal + rhetorical-subgoal prompts; or a persuasive goal + content-subgoal prompts. Rhetorical subgoals increased text quality, variety of rhetorical moves, number of complex propositions, and classification knowledge. Content subgoals increased the number of simple propositions in text. A path analysis indicated that content-subgoal prompts and rhetorical-subgoal prompts elicited different paths to writing and learning.