I had a skype interview with one school that mentioned that they would be contacting people about an interview weekend starting January 31, but never explicitly invited me. Then the next day, I received a formal invitation for an interview on February 1, but with a note that not attending because of financial issues or problems with missing school would not affect my chances of being accepted.
In my situation, both schools were about equal ranking, but I had already skyped with the professor at the first, and had never been able to speak with the professor at the second. I waited a few days, and my dad gave me the same advice: to tell the second school that I was very interested in attending, but needed to make sure I could work it out in my schedule, and to tell them that I would let them know in the next 2 or 3 days.
I think this is a very appropriate way to handle it, but I decided to just accept the second school's offer without even sending them an email to buy me some time, and still haven't heard back from the first school. Although it worries me that I was never invited to the interview weekend mentioned during my skype interview, at least I didn't have to choose between two schools. From what I have seen online, it seems like everyone has interviews these first few weekends, so the schools must expect that other offers might come up, in fact at another school, my POI told me they really try to get their invites out before other schools so that students don't have other interviews already scheduled. That being said, schools may suspect that if you make it seem as though you are waiting for some outside factor before agreeing to attend, that you might not put them at the top of your list.