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. We studied how accurately visual lines and concave/convex surfaces let users draw 3D shapes attached to physical vs. virtual objects in AR. Results show that tracing physical objects is 48% more accurate, but takes longer than tracing virtual objects. Concave physical edges further improve accuracy due to their haptic guidance. Our findings provide initial metrics when designing AR sketching systems.