I have three possible referees: Postdoc-1 who'd write a great reference, the Professor who'd write a good enough reference if Postdoc-1 tells him to, and Postdoc-2 from another lab, who's happy with my work but who's a "man of few words", i.e. would write I'm good but not more.
Obviously, I'll go for the professor, but which one of the others? Would it be better to have two independent accounts of me being ok, than two enthusiastic persons from the same place?