Thanks for the advice everyone. I actually like both schools, and I want to have acceptances from both before I make the big decision. Last year I was only accepted to one school and it turned out not to be a good fit, so I was left with no choice but to decline. That being said, school B does seem to have a better program, so it will probably be my first choice unless something at the interview visit convinces me otherwise.
As for your questions, katalytic, in my experience the interviewers like to ask about your past experiences and future interests to determine your "fit". Basically, they want to know if what you want to do is related to their strengths as a department. For these questions, try to "spin" your answers so that you emphasize these aspects when you are discussing your past research/future intended research, even if they are not perfectly in line with what that school does.
I have never encountered a "if we give you a spot, will you come here" situation, and I really don't think that's a fair question to ask on their part. I think it is expected for prospective students to apply to more than one school and be given a chance to look at all options. I would be wary of any school that puts you on the spot like that.