Ric... something is wrong. My guess (uneducated at best...) is that when your bags are filled and weight is on them, it stretches the bags out enough that one of the bags is rubbing against something that eventually tears through the bag. Was it the bag on the same side both times?
Firestone airbags are pretty good quality. One failure could be explained away by manufacturing error or some type of road hazard. TWO failures is either an amazing coincidence or else there is something rubbing a back and causing the issue.
If you just decide bags aren't right for you, Timbrens are an option, Roadmaster Active Suspension (RAS) is an option, custom leaf springs, helper coil springs etc are all options for you as well.
Good luck and let us know what you decide!