I've narrowed my potential research for this upcoming post-M1 summer to two labs, and I'm having trouble making a decision. I have a history with lab A, and if I continue there, I'll likely complete a substantial project, get a nice rec letter from an up and coming MD faculty member, and get published. Lab B is in a field related to my past research with lab A, but in a slightly different direction where I would learn new lab techniques and analyze datasets I've never worked with before. I'll likely also complete a project and publication. Looking at the projects alone, I'm slightly more interested in lab B because of all the new exposure I'd get, though I'd be happy in both labs. The only drawbacks to lab B seem to relate to rec letters. Because I don't know the PI of lab B as well, the letter may not be as good, though by residency application time, this might not be an issue. The other issue is that the PI of lab B isn't an MD (just a PhD), and I'm not sure if that's an issue with residency applications. Both projects are translational research, so they're very relevant to medicine.
Am I overthinking the rec letter issue?
Am I overthinking the rec letter issue?