First of all, congrats on getting an interview.
Whether you interview on campus with an adcom (2nd year student) or locally with an alum really shouldn't make any difference to your chances. Booth keep repeating that message and if you look through the Booth applicants thread there doesn't appear to be any correlation between on campus interviews and successful applications.
In my case, I flew from India to interview on campus because I wanted to visit the school and compare it to other top b-schools. You do have to pick up all your own costs but if you book far enough in advance it's not horribly expensive. I feel that it helped my interview because I was able to talk about "why Booth" in much more detail (I had an afternoon interview so everything was still fresh in my mind). On other other hand, I know several people who interviewed with an alum having never visited the campus and got in as well.
I think that if you're really confident about what Booth is looking for in its students and you're clear about "why Booth", then perhaps you don't need to visit the campus. On the other hand, if there is any doubt in your mind then this is your last chance to research the school.