Pervasive networks formed spontaneously by wireless mobile devices capable of ad hoc communication make it possible to get services from these devices, and to compose them dynamically so as to provide people with new and more sophisticated application services. Discovering, invoking and composing services in such networks are challenging tasks, due to the numerous and unpredictable connectivity disruptions caused by mobility and the short radio range of wireless interfaces. By implementing the " store, carry and forward " principle, oppor-tunistic networking and computing can help tolerate connectivity disruptions and support communication in such intermittently-connected networks. In this paper, we present a system we have designed to discover, to select, to invoke and to compose REST services dynamically in opportunistic networks. This system implements both orchestration-based and choreography-based composition strategies. In this paper, we compare these two techniques in two different scenarios: one involving nomadic people roaming in an open area, and another one involving people attending a sport event.