I was originally an undergraduate student from pure mathematics and am planning to transition into biostatistics/bioinformatics/statistical genetics. To accumulate experience, I planned to apply for some internships but lots of them require 2 recommendation letters. I have, for sure one of them from my master thesis supervisor. For another one, I am choosing from a Ph.D. student or an assistant professor.
For the assistant professor, I took a 1-month summer project in applied mathematics (ecology) with him. It has nothing to do with biostatistics/bioinformatics/statistical genetics except some programming experiences. I met him once per week face to face.
For the Ph.D. student, I am currently working under him on a topic related to biostatistics/bioinformatics/statistical genetics (collaboration), which will possibly lead to publication. But still, the topic is not directly related to the internship project. I had never seen him face to face due to his different geographical locations.
Who should I ask recommendation letter from? Any suggestions?