This is just my opinion, but I think math research would look pretty impressive.
Most people with research has the most basic, cookie cuter "I did one semester of research and nothing published" type of experience. Anything that is meaningful, shows you have knowledge of the research process, and contributed enough to present a poster/ publish will have more weight.
The chances that you'll continue your undergrad research topic as a physician are minuscule, so the point of the process is to establish research methodological skills and learn how the process works so you can continue with other subjects in the future.