Modern software operates in highly dynamic and often unpredictable environments that can degrade its quality of service. Therefore, it is increasingly important having systems able to adapt their behavior. However, the achievement of software adaptability can influence other software quality attributes, such as availability, performance or cost. This paper proposes an approach for analyzing tradeoffs between the system adaptability and its quality of service. The proposed approach is based on a set of metrics that allow the system adaptability evaluation. The approach can help software architects to guide decisions on system adaptation for fulfilling system quality requirements. The application and effectiveness of the approach are illustrated through examples and a wide set of experiments carried out with a tool we have developed.