On-Campus vs Alumni Interview ?

Most schools have a boilerplate statement to the effect that the status of the interviewers (staff/students/alumni) has no effect on the weighting of interview in the decision process. Nonetheless, given the choice, I would prefer a admissions staff interview over a student interview or an alumnus/a interview.

In your case, of course, the staff interview is not an option. One advantage of the on-campus interview is that participating in an on-campus interview makes it obvious that you are interested enough in the school to see it in person. When I do mock interviews one of my favorite questions is to ask students whether they have toured the campus and ask follow-up questions why they have failed to do so if they live in the region.