The safety factor is why I would shy away from aftermarket ones, you are literally betting with your life.
If you are afraid of the factory webbing, it would not be hard to simply unwind the fabric from the retractor and replace just that part. Any upholstery shop should be able to sew the ends for you.
If the belts are in good condition they in all reality will last many more years though. I can't remember actually seeing a seatblet that broke in an accident. The webbing is just to wide. They really are much wider than needed for stress, the width is to support your body durring a crash and not cut you in two like a rope would.
