In this paper, we describe a user study aimed at evaluating the effectiveness of two different data visualization techniques developed for describing complex environmental changes in an interactive system designed to foster awareness in sustainable development. While several studies have compared alternative visualizations, the distinguishing feature of our research is that we try to understand whether individual user differences may be used as predictors of visualization effectiveness in choosing among alternative visualizations for a given task. We show that the cognitive ability known as perceptual speed can predict which one of our target visualizations is most effective for a given user. This result suggests that tailored visualization selection can be an effective way to improve user performance. Author Keywords Evaluation of visualization techniques; individual differences. ACM Classification Keywords H5.m. Information interfaces and presentation (e.g., HCI): Miscellaneous.