I did a research project, in which I was advised by two researchers. Most of the time, it was a PhD student who defended and transitioned to post-doc during the project. But my official advisor, with whom I talked less often, was this post-doc's own advisor, hence more established and with a "full researcher" position.
I am not sure which researcher I should contact about letters of recommendation for PhD applications. They both gave me good feedback and advice. It was the senior advisor who graded me (close to a perfect grade), but as I said, the post-doc advised me more often.
Note that I am not yet applying to anything, I just want to ask if they would be willing to write recommendations for me in the future.
Would it be better to contact the post-doc, the full researcher, or both in a joint e-mail? Does the job title make a big difference, or is it more a matter of how closely they followed the project?