Should I contact a PhD student or his supervisor for potential collaboration

I find some recently published papers which are related to my research and am interested in contacting the authors for potential collaboration. However, in those papers, the first author is a Phd student while the second author is his supervisor, who is a well-know professor in my field. In my field, the order of author is based on contribution, which means the Phd student contributes more to the paper than his supervisor.

Hence, I don't know whether I should contact the student or his supervisor to initiate the collaboration. I can see that the advantage for contacting the student is that he should know more about the research than his supervisor. On the other hand, I also looks for someone that can write a reference letter for me in the future, and the supervisor sounds better.