Which sentence is most clearly written?

The first sentence conveys a clear sequence of events: it establishes the context that Mary is only going to have lunch with Sue during her next visit to town. This structure emphasizes the timing of when the agreement to have lunch occurs, making it easy for the reader to understand that the lunch plans are contingent upon Mary being in town.

In contrast, the second sentence combines the agreement and the condition of being in town in a single phrase, which can create a slight ambiguity regarding the timing of the lunch in relation to Mary's availability. While it is still understandable, the clarity and emphasis on the conditional nature of the lunch plan are more pronounced in the first sentence. Thus, the first option is considered the more clearly written choice.