I need to get some more 2 1/2" cardboard flips for my chips and I was wondering if there are different types of plastic used in the windows and if there is one type that is better than others? I noticed that some of the flips that were used a long time ago from the collection that I bought are sticking to the pages they were in. Some of the flips I am looking at are made of polypropylene and others say mylar. Is one better than the other?