It really shouldn't matter as long as you make a good impression. As a general rule, I would say if you can swing an on-campus interview with someone on the committee, you are better off, but if it's inconvenient, sometimes an alumni interview can work in your favor if you get lucky with the right person. Go for the adcom interview first if you can, though.

I'm in a similar dilemma, except that if I schedule an on campus interview, it would most likely be with a second year student that helps with the Adcom. To me, I would only want to make the trip if I could interview with an employee in admissions, not a student. I'm leaning toward an alumni interview in my city. Any thoughts/suggestions?

I would DEFINITELY go with admissions person over an alum.

The admissions consultant IS one of the people who will be deciding on your application so you want to talk to the source. If you can get this adcom person on your side and get them to become your champion; someone who will fight for your admission, then you will have a much greater chance of getting accepted. The alum, who will just write in on your behalf (you and probably a couple of other names), will have a much larger disconnect to the adcom. Possibly the admissions person doesn't like that alum or thinks that alum always gives nice reviews...then what do you have?

Impressing the adcom will be a big advantage. Let me know if you have any further questions by PMing me.

Thanks!