As the mobile Internet continues to grow, there is an increasing need to provide users with effective search and information access services. In order to build more effective mobile search services, we must first understand the impact that various interface choices have on mobile users. For example, the majority of mobile location-based search services are built on top of a map visualization, but is this intuitive design-decision the optimal interface choice from a human centric perspective? In order to tackle this fundamental design question, we have developed two proactive mobile search interfaces (one map-based and the other text-based) that utilize key mobile contexts to improve the search and information discovery experience of mobile users. In this paper, we present the results of an exploratory field study of these two interfaces - involving 34 users over a 1 month period - where we focus in particular on the impact that the type of user interface (e.g. map vs text) has on the search and information discovery experience of mobile users. We highlight the main usage results - including that maps are not the interface of choice for certain information access tasks - and outline key implications for the design of next generation mobile search services.