Help choosing last LOR

[I_mix]

Hey guys, 

I just need some opinions about choosing a final LOR writer. 

My first two are basically set in stone: 

1) My thesis supervisor (for obvious reasons)

2) A prof for a class that was very quantitative, and I'm intending to send in my final project (expanded and with edits) to try and get it published.

I'm pretty torn about the last one. I have two options, as of now:

1) A previous adviser under whom I did an undergraduate literature review with. Their work is somewhat relevant to what I intend on investigating for my graduate research. However, I haven't spoken with them since 2013/2014. 

2) A joint letter of recommendation from the professor of two of my more recent courses, which involved field work and a paper, and the lab instructor for one of said courses. This would be valuable in that I TA one of her courses and work with the lab instructor.

My struggle here is that, by asking my previous adviser, it would show that I was interested in my field of study for a long time. However, the second recommender would allow me to show more facets aside from research skills.

Any and all advice would be appreciated.

[fuzzylogician]

It sounds like (2) is the stronger letter. I would choose that. There will be ways of discussing your long-standing interest in your field, for example by discussing this undergraduate project in your SOP. 

[I_mix]

There's one caveat here that I forgot to mention: That "professor" hasn't obtained their PhD yet and the lab instructor doesn't have a PhD either. Will that work poorly for me? Or does that not matter as much since I have two recommenders that can attest to my research skills and ability?

 

[fuzzylogician]

Yeah, that will make the letters less impactful. Someone who doesn't have a PhD can't really speak to someone else's ability to complete such a program, and won't have (m)any students to compare you to. They often won't have a track record of placing students in graduate programs who go on to be successful. This means that adcoms won't be able to rely on the opinion of the writer as much as they would in the case of letters from more experienced folks. But if this is the best option you have and it'll be a detailed letter from someone who knows you well talking about aspects that other people might not touch on, and you have two other letters from more experienced professors, you should be ok. 

[Yanaka]

Advice I received from a prof at Princeton is: those teachers are fine, but get a tenured prof to endorse their letter.