Employing social robots in public spaces to support employees at work is a frequently discussed scenario. However, the success of robotic systems often depends on people’s willingness to initiate interactions with them. This makes understanding people’s usage decisions crucial, yet only limited research has been done on why people select publicly accessible social robots over alternatives, such as human employees. Amongst various factors, people’s diversity characteristics are likely to influence this decision, such as people’s locus of control when using technology and their self-efficacy in human-robot interaction. To investigate this choice for or against using a robot, a field study (N = 65) was conducted in two public libraries in the Ruhr area (Germany). Participants had to decide to interact with a robot or an employee and were subsequently asked to explain their decision via a questionnaire and an interview. Results reveal that the decision could neither be explained by people’s locus of control when using technology nor by other diversity characteristics. Furthermore, no significant differences in self-efficacy in human-robot interaction between users who chose the robot instead of the human employee were found. Finally, the qualitative findings point to general interest in robots and people’s differences in dealing with novelty as reasons for their choice. Overall, our findings offer insights into the decision for or against the usage of a robot, which are relevant to both, research and the deployment of social robots in public spaces.