In the early stages of design, several concepts are usually generated to explore the possibilities. This paper investigates how well a computer-based system can support design thinking. SESAME is a novel 3D design system that aims to support creativity during the explorative phase of the design process. We report an evaluation comparing SESAME to paper sketching for early design exploration in an urban design scenario. Through the user evaluation, we illustrate how important it is to support essential properties of traditional sketching, such as rapid creation/modification, emergent shapes, and tolerance to ambiguity. Additionally, we show that a 3D system can indeed facilitate form exploration at the early stages of design thinking.