I was recently asked whether it was better to have a strong letter of recommendation for graduate school from a professor in a different major (who has observed and supervised your academic work), or a positive but not strong letter from a faculty person in your major.
Personally, I would go with the stronger letter, even though the faculty person is from a different discipline. Letters of recommendation carry a lot of weight when it comes to applying to graduate programs. Ideally, you will have 3-4 letters written by faculty in your major. However, if a professor from a different discipline has observed your academic work, (perhaps you switched majors, completed a minor, or conducted research in a different field), then their letter would be preferable to a lukewarm or “wishy-washy” letter from a professor in your major.