BTW Rodney, length of chains is spot on but not the only thing by far. The resins chosen have enormous influence over the strength of the cured epoxy. They choose the different components to influence the desired characteristics. What they put into the two tubes for 5 minute was chosen to give the strength and bonding (two different characteristics) they intend to market. It won't be the same components they put into 15 minute. Very often the quicker epoxies will exhibit far more flexibility when cured than the longer cures. But that still depends on what solids are in each and their rigidity etc.

As for modeling, one thing that will help save weight and insure adequate strength of whatever we epoxy, is how much wasteful overage we have after the epoxy has cured. We should always try to use as little of whatever epoxy we've chosen. Take the time to coat end grain if possible, and wipe away excess when you can.

BoiseBee, if you're looking to build faster without sacrificing strength, there isn't really much time to be gained between fast and slow epoxy. Thick CA however will save time and will often give far more joint strength as it will penetrate like slow cure to a great degree.