Polypropylene does not last longer than nylon and it only has a fraction of the breaking strength and very poor abrasion resistance when compared to nylon. Polyester webbing on the other hand is nearly as strong as nylon and does weather better in the sun than nylon. Any webbing that is dyed a dark color, especially black, will have improved UV resistance. You will notice Ortlieb uses black nylon for all their pannier straps.