I'm currently a senior in high school and plan to get a masters in Biomedical engineering. If i want to work in prosthetics, would mechanical engineering or materials science be a better undergrad degree?

I am big believer in passion. So the question is more for you then for me. If you are interested in designing the prosthetic, including the mechanism, how it is produced, how it attaches to the patient. So if you are interested in materials, there is a lot you can do with material design, selection, biocompatibility, chemical resistance, structural integrity, etc. Some prosthetics have electronic and software needs as well. Biomedical Engineering is also a degree that let's you bridged the biology with the traditions engineering disciplines.

The short answer is that medical companies have a need for a lot of different talents in engineering. You should focus on the areas that you have the highest passion for and find ways to apply it in the field of interest. For your electives, you can take courses that will make you better rounded for the career path you wish to follow. More knowledge is a good thing.

I would recommend mechanical engineering. I can't imagine that there is a huge choice for materials that can be used for prosthetics. At any rate, you can take additional materials courses in either undergrad or graduate school. I took at least six materials courses earning my masters.

As someone who went into college with the same desire to work on prosthetics, I would highly recommend the path of a Mechanical Engineering degree supplemented with elective work in biomedical sciences. I graduated with a Biomedical Engineering master's degree and found most medical device companies much more interested in hiring Mechanical and Electrical engineers who had expressed interest (sometimes not even that) in working on medical technologies.