This paper describes a user study about the influence of efficiency on modality selection (speech vs. virtual keyboard/ speech vs. physical keyboard) and perceived mental effort. Efficiency was varied in terms of interaction steps. Based on previous research it was hypothesized that the number of necessary interaction steps determines the preference for a specific modality. Moreover the relationship between perceived mental effort, modality selection and efficiency was investigated. Results showed that modality selection is strongly dependent on the number of necessary interaction steps. Task duration and modality selection showed no correlation. Also a relationship between mental effort and modality selection was not observed.