Users tend to gravitate more to zippered doors because they are great at keeping bugs out as well as critters. The downside to zippers, however, is that when subject to the elements they can become weak, and over time they may not work properly- leaving you wide open and unable to completely close your doors or windows. Drawstrings save users a little bit of money, and they don’t run the risk of breaking over time. The downside to this option, however, is that they won’t work as well at keeping out bugs and critters they way a zipper could.