Stock cap says 16 lbs, BUT even though the aftermarket caps might only state 13 lbs, they have a 13-16 lb RANGE, which is likely true of the stock cap as well.
The biggest difference is most of not all of the aftermarket caps have the additional clamp/latch mechanism, unlike the stock screw-on design.
For the price, I'd just buy a new Motorcraft rather than bother with aftermarket for the $1 savings.