Agile and traditional plan-based approaches to software system development both agree that prioritizing requirements is an essential activity. They differ in basic strategy - when to prioritize, to what degree, and how to guide implementation. As with many software engineering methods, verifying the benefit of following a particular approach is a challenge. Industry and student/classroom based experimental studies are generally impractical to use for large numbers of controlled experiments and benefits are difficult to measure directly. We use simulation to validate the fundamental, yet typically intangible benefits of requirements prioritization strategies. Our simulation is directly based on detailed empirical studies of agile and plan-based requirements management studies. Our simulation shows, as many have claimed, that an agile strategy excels when requirements are highly volatile, whereas a plan-based strategy excels when requirements are stable, and that there exist mixed strategies that are better than either for typical development efforts.