I can get a letter from a course instructor that I got a highest possible grade. He is a very well known professor at a top 5 institute. Of course, he can say little about my research.
I can also get a letter from my research collaborator who just got his PhD from a not-so-famous school. We worked on three papers so he can definitely comment my research in details. He is an assistant professor in a research-focused school rank at 100.
Which one is better for a research (PhD) program?
The program I am applying to requires two letters. The first letter is from a big name who I've done research with.