I'm having trouble trying to pick between two options for my third LOR. I already have 2 professors willing to write me letters with whom I have done research (a professor whose lab I've worked in for over a year and an assistant professor at another school where I had a summer research internship). I can't speak for how good the letters from these two will be.
For the third letter, I am getting mixed advice. At first, I was going to ask a professor with whom I have had a few classes; he knows me somewhat well, but only my academic ability. He wrote me LOR for the internship, but I don't remember it being particularly strong (he really only touched on my academics, but I've had another class with him since; maybe he'd write a stronger one this time?). Some people are telling me to skip the mediocre letter from a tenured professor and ask the postdoc with whom I've been doing research (knows me really well, would likely write a really good letter for me, has had his phd just over a year now). Others are telling me a postdoc letter would reflect poorly upon me, but I'm sort of leaning toward a strong letter vs an average one..
Any advice?