As others have said, there are pros and cons to each.
One important thing to consider that hasn't been mentioned is that an established tenured professor may be more willing to throw a first-authored publication your way. Or, even moreso, less likely to "claim" a first authorship that is rightfully yours.
You want to make sure that the junior faculty member realizes that a student's success is just as much his/her success - sometimes that's hard for new faculty to grasp, especially when they are just coming out of postdoc.
However, just as I have seen junior faculty who maintain the cut-throat postdoc mentality, I have met others who are very supportive and nurturing. I would go with your gut, and how well connected each person is. Even junior faculty have meaningful connections that can make a difference in your career (e.g., their own advisors, internship experiences, etc.)