Unable to find a reference that distinguished one being better than the other, I called Barnett and was told that the Kevlar surfaces lasted longer and gripped better. Call them to see if it's the right choice for your app- (805) 642-9435. Soon after, I stumbled across an ebay auction for the Barnett Kevlar extra plate kit from the Shreveport, LA HD dealer and got it for $99. too good to pass up for my 95" build.
If you're adding power and are concerned about slippage, the Barnett heavy duty spring is relatively cheap and since you have it dissasembled anyway.... But remember that it's going to result in a stiffer pull. I added the White Bros/ Burly Easy Pull extension. Also from ebay, for $11. Longer pull, but lighter than stock, even with the heavier spring.
Best of luck with your choice and don't forget to check your chain tension while you're in there and use a new primary gasket. Murphy is out there and will penalize you for failing to pay for an overpriced gasket by leaving you with an incurable seep onto the wrinkle paint along the topside and drip on the floor.