Using social robots and virtual agents (VAs) as interfaces for health monitoring systems for older adults offers the possibility of more engaging interactions that can support long-term health and well-being. While robots are characterized by their physical presence, software-based VAs are more scalable and flexible. Few comparisons of these interfaces exist in the human-robot and human-agent interaction domains, especially in long-term and real-world studies. In this work, we examined impressions of social robots and VAs at the beginning and end of an eight-week study in which older adults interacted with these systems independently in their homes. Using a between-subjects design, participants could choose which interface to evaluate during the study. While participants perceived the social robot as somewhat more likable, the VA was perceived as more intelligent. Our work provides a basis for further studies investigating factors most relevant for engaging interactions with social interfaces for long-term health monitoring.