I'm probably the exception, but my bleached ones are much softer than my unbleached. This is after several years of use. I might be wrong, but I think I've read that the way some pfs are bleached is not harmful, as long as it's a peroxide based method as opposed to chlorine. I could be wrong though...
Copied and pasted from Green Mountain Diapers
White Prefolds Our white diapers are hydrogen peroxide bathed, NOT chlorine bleached. Hydrogen Peroxide basically the same thing as the main ingredient in OxiClean. It's dioxin-free and acid-free, known as "natural bleaching process". Thus, these diapers last longer than their chlorine-bleached counterparts which we do not carry. We encourage most people to choose white, because often people get frustrated by the number of hot wash cycles which are needed to get the natural cotton oils off the unbleached ones. With the natural bleaching process, that "degumming" is already done safely for you, but with unbleached, you need to do that yourself. They are the same diapers otherwise.