I'm applying for several astronomy REUs, for which you can have 2 letters of recommendation. I have three faculty members in mind:
1) Engineering physics professor I've known for many years personally, and is the PI for the lab I've worked in for a year. I haven't taken his class yet, but he seems to have a good opinion of me.
2) Astronomy professor whose class I took last semester. He knew me by name early in the semester since I asked good questions during and after class and participated a lot, although I always had to work during his office hours. I got an AB in his class.
3) Engineering technical presentations instructor who was impressed by my projects. He told me at the end of the semester that he would be happy to provide a strong recommendation should I ever need one. However, the programs are for astronomy and he teaches courses on writing/speaking. And he's a faculty associate rather than the other two professors.
My instinct is to ask the two professors, but I'm not sure how much professor #2 would be able to say about me. I'm especially uncertain because #3 would write a very strong letter, albeit more lightly weighted. Any advice?