Another point to consider, though, is how much experience, if any, this person has had writing LORs. I was considering asking an associate poli-sci prof for a letter, and it was suggested to me that oftentimes younger/newer individuals simply aren't aware of the particular tenor that is appropriate for a LOR. Not that such an individual wouldn't or couldn't write a great letter, and if this professor knows you well it seems an excellent choice--but you might want to be more forthcoming about exactly what sorts of things should be addressed.