We analyse the conversational and pedagogical behaviour of AI and human tutors in one-on-one task-based, text-based tutoring and measure their respective effects on learners' engagement, perceptions, and writing fluency. The study follows a crossover longitudinal experiment with 50 A2-B1 learners of French and records their interactions with human tutors and AI tutors. The learners' perceptions, engagement, and writing fluency are measured through surveys and keystroke logging.