Augmented reality (AR) offers promising opportunities to support manual assembly, but there is little consensus on how much information AR instructions should contain, reflecting debates between cognitive-load-oriented minimalism and multimedia-learning-based benefits of redundancy. These debates manifest in practice as rich, multimodal overlays or minimal, complexity-adaptive visualizations designed to avoid clutter and ease authoring. This study compares these approaches by contrasting a redundant AR concept combining three-dimensional models, photographs, and videos with a minimal concept that adapts visualization types to assembly step complexity. In a between-subject experiment with 30 participants (mixed-experience; heterogeneous backgrounds) performing a heat-pump assembly task for the first time in a spatially constrained setup, errors, task time, workload, and usability were measured. The redundant concept led to significantly fewer errors and a lower per-step error probability than the minimal concept, without a penalty in assembly time. Workload and usability were comparable across concepts and primarily driven by performance rather than by visualization style. Step complexity strongly predicted completion time but not error rates, suggesting that operators slow down on complex steps while failures are more sensitive to instructional design. These findings suggest that overly minimal AR instructions increase error risk, whereas redundant AR instructions stabilize performance.