Advice on letter of rec writer? Forum

[jeriatrik]

During undergrad, I did STEM research in a biochemistry laboratory for 3 years under my graduate student mentor and our professor/principal investigator of the laboratory. I recently asked my graduate student mentor, who has now graduated and has his own doctorate, for a letter of rec. I decided to ask him and not my professor because he knows me better and we worked directly with each other on research projects, whereas my professor was more secondary to the situation.

However, he wants to write most of the content of the letter and have our professor sign off on it because he's a very distinguished scientist. I'm not sure what to do, should I agree with him? Is prestige that important? Would a former graduate student's (now PhD) letter be worth less than an established professor? Thanks.