There are two alternative display metaphors for Augmented Reality (AR) screens: a see-through window or a magic mirror. Commonly used by task-support AR applications, the see-through display has not been compared with the mirror display in terms of user's s task performance, even though the `mirror' hardware is more accessible to general users. We conducted a novel experiment to compare participants's performance when following object rotation cues with the two display metaphors. Results show that participants's overall performance under the mirror view was comparable to the see-through view, which indicates that the augmented mirror display may be a promising alternative to the window display for AR applications which guide moderately complex three-dimensional manipulations with physical objects.