Besides sketching in mid-air, Augmented Reality (AR) lets users sketch 3D designs directly attached to existing physical objects. These objects provide natural haptic feedback whenever the pen touches them, and, unlike in VR, there is no need to digitize the physical object first. Especially in Personal Fabrication, this lets non-professional designers quickly create simple 3D models that fit existing physical objects, such as a lampshade for a lamp socket. We categorize guidance types of real objects into flat, concave, and convex surfaces, edges, and surface markings. We studied how accurately these guides let users draw 3D shapes attached to physical vs. virtual objects in AR. Results show that tracing physical objects is 48% more accurate, and can be performed in a similar time compared to virtual objects. Guides on physical objects further improve accuracy especially in the vertical direction. Our findings provide initial metrics when designing AR sketching systems.