Wi-Fi and LTE standards define several traffic classes to prioritize applications based on their requirements. When these technologies coexist in unlicensed bands, the class selection of one system impacts the performance of the other. In this paper, we investigate how the traffic class selection affects the delay for completing the transmission of a fixed number of bits. We develop an analytical framework which characterizes the average delay under Wi-Fi/LTE coexistence. Our framework allows us to optimize the class selection for a Wi-Fi or LTE station based on the traffic class selected by the surrounding stations and minimize the average delay. We show that operating at a high priority class does not always minimize delay. Under certain contention and class selection conditions, a low priority class reduces the collision probability while increasing the airtime once the channel is captured. This leads to a lower overall delay. We provide numerical examples that demonstrate the inherent tradeoffs between the traffic class parameters.