In order for natural interaction in Augmented Reality (AR) to become widely adopted, the techniques used need to be shown to support precise interaction, and the gestures used proven to be easy to understand and perform. Recent research has explored free-hand gesture interaction with AR interfaces, but there have been few formal evaluations conducted with such systems. In this paper we introduce and evaluate two natural interaction techniques: the free-hand gesture based Grasp-Shell, which provides direct physical manipulation of virtual content; and the multi-modal Gesture-Speech, which combines speech and gesture for indirect natural interaction. These techniques support object selection, 6 degree of freedom movement, uniform scaling, as well as physics-based interaction such as pushing and flinging. We conducted a study evaluating and comparing Grasp-Shell and Gesture-Speech for fundamental manipulation tasks. The results show that Grasp-Shell outperforms Gesture-Speech in both efficiency and user preference for translation and rotation tasks, while Gesture-Speech is better for uniform scaling. They could be good complementary interaction methods in a physics-enabled AR environment, as this combination potentially provides both control and interactivity in one interface. We conclude by discussing implications and future directions of this research.