That's a tough one! I think if you let them know you are waiting on other schools before you decide you are pretty much killing your chances of admittance. That being said, it would be rude to agree to attend and then back out if they have spent any money on preparing for your visit. So I suppose it depends on how much preparation the school goes though for their interviewees. If you do get a better offer for the same weekend and decide to take it, you are also probably going to lose out on the first school, because asking for a Skype interview instead is sending a clear message of their ranking on your list, and schools don't generally want to offer one of their limited admittance spots to someone who has made it clear the school is a backup choice.
I suppose if it were me, I would tell them I plan on attending but may have a scheduling conflict. Then if I got a better offer for the same weekend, I would decline to be interviewed by the first school.