In recent years there has been considerable research into the development of mobile context-aware applications. The canonical example of such an application is the context-aware tour-guide that offers city visitors information tailored to their preferences and environment. The nature of the user interface for these applications is critical to their success. Moreover, the user interface and the nature and modality of information presented to the user impacts on many aspects of the system's overall requirements, such as screen size and network provision. Current prototypes have used a range of different interfaces developed in a largely ad-hoc fashion and there has been no systematic exploration of user preferences for information modality in mobile context-aware applications. In this paper we describe a series of experiments with multi-modal interfaces for context-aware city guides. The experiments build on our earlier research into the GUIDE system and include a series of field trials involving members of the general public. We report on the results of these experiments and extract design guidelines for the developers of future mobile context-aware applications.