Spatial diversity and multiuser diversity attract much research interest recently. In this paper, through asymptotic analysis of the average system capacity and scheduling gain, we investigate the cross-layer interaction between these two forms of diversity in wireless networks. Rigorous proofs and necessarily stronger results in terms of convergence are provided for some intuitions in this area. Equally important, explicit expressions of scheduling gain and average system capacity in various circumstances that reveal interconnections and fundamental tradeoffs among key system parameters are given, which afford us some insights in system design. Our results are general enough to cover many practical scenarios of interest.