I need 3 letters of recommendation to apply to PhD programs in Astrophysics. I've done research with 2 professors, so will be asking both of them for letters. One of these professors -- my main advisor -- also taught a course I took (Cosmology), and I got an A. However, I'm not sure who to ask for the third letter.
One option is the advisor for undergraduate physics majors at my school -- I have talked to her about various things here and there, and she knows my grades, and that I am involved in a lot of leadership activities in the Physics Department. But since she is not a professor and can't comment on my research, I worry this might be a bad choice.
Another option is to ask a professor that simply taught a course I did well in. For example, I got an A+ in the Computational Physics & Astronomy Lab. However, I rarely spoke with the professor, so all he would know about me is my grade, that I always showed up to class (a lot of people didn't), and whatever else I tell him (maybe I could schedule a meeting with him to tell him a bit about myself and give him my CV).
Which do you think is the better option?