When players who choose a common strategy face a common shock, while players who choose different strategies face independent or imperfectly correlated shocks, equilibrium choices exhibit differentiation (respectively emulation) when the sign of the cross-partial derivative of the firms' profit functions with respect to the realizations of the uncertain variables is negative (respectively positive). I consider a variety of applications, including technology choice, brand investments, and R&D races, many of which can be characterized as two-stage games. In such games I demonstrate that differentiation is more likely to occur when the second stage game is a game of strategic substitutes.