First, when you say the "F" word (fold) I cringe.. You do not want to store them folded (just incase I did not understand). Both are plyable. If you fold the mylar it is ruined, if you fold the PVC it will be OK (once flattened out and the wrinkle relaxes). The Mylar will bend and flex easily and will tend to remain flat, the PVC is soft. Note: I would NOT put a High End note in PVC for risk of folding. High end notes go in HIGH END holders. It sounds like your wife's notes are mainly circulated stuff, the Mylar being stiffer will "Iron Out" many of the smaller wrinkles over time and IMPROVE the looks of your notes.

PVC= Soft
Mylar= Rigid