Letter of Recommendation dilemma

[statfan]

Hello, I am in dilemma of choosing references for (bio)statistics PhD applications and I am kinda in need of your help. I am currently working towards my master's thesis and I got one letter from my supervisor. I also got a letter from a very well-known biostatistician who I took graduate survival analysis with, although he knows me less well. Now I am debating on who I should ask for the third letter. Currently I have two options: An assistant prof. that I took two undergrad statistics courses and did research with during my undergrad or an associate prof. who I took grad statistical inference and experimental design with. The undergrad research did not yield significant results and since I already asked two letters from biostatistics faculties, I am more leaning towards the associate prof. who can attest to my math/statistics skills. However, some of my friends told me that I should get as more letters as possible from people that I did research with and I am really confused now. If I mention my undergrad research in my SOP but did not ask a letter from the assistant prof, would this be looked upon unfavourably? 

Edited October 3, 2018 by statfan

[Stat Assistant Professor]

I think the associate professor who can attest to your quantitative skills is the better choice. Go with your instinct.