Ask HN: Do you hire for malleability or past experience?

By "malleability" I mean a person who is not an expert in a field but his/her past experience shows varied skillsets and demonstrates flexibility of picking up new technologies and domains. Few years ago I read an article by Joel Spolsky on hiring. He gave an example: Say you have a requirement for PostgreSQL database engineer. And you get two applicants, one is a rock star PostgreSQL engineer who has many years of experience and the other applicant has experience but the CV shows that applicant has done lot of projects in different fields and technologies, but not much experience in PostgreSQL. So which applicant would you hire? The one who is a rockstar PostgreSQL engineer or the one who has little experience with PostgreSQL but has worked on various types of projects?