Kevlar belts ARE stronger than steel belts, assuming the engineering quality is there.
I'd be surprised if the stock belt isn't already Kevlar, though. That seems like a no-brainer these days. But if the "slap" goes away the aftermarket belt must be less prone to stretching.
I do know that when I first bought my 1999 Volkswagen diesel the cam drive belt had a 40,000 mile life expectancy, and field experience among the less mechanically inclined indicated that you could expect failure and engine destruction in less than 50,000 miles. A better belt, either with Kevlar or with MORE Kevlar (we never figured out the exact difference) was developed that raised the change interval to 80,000 miles. Kevlar is the real deal.