what school is in question? Generally alum interviewes tend to be less formal, more conversational and when i was in yoru shoes, I oten received advice to go with alum interviews if possible.
In my personal experience student interviews have been the worst because students generally are not that good at interviewing, often are too rigid in following their list of questions, and I not great listeneres/ not good at picking up subtle messages (I was often asked a question that I felt I had already answered indirectly and a better listener would have picked up on)
If you can get an interview with a memebr of adCom and you are good at interviewing, than take that over an alum; otherwise I'd probably go with an alum. Schools weigh equally alum and student interviews and if you already visited that school on a separate ocasion than you have nothing to worry about.
so in my ever so limited experience:
AdCom = great interviews, they know what they are doing and can "fight" for you come decision time if you impress them
Alum = good interviews, not clear how important in the decision making
Student = weak interviews,not clear how important in the decision making