I don't think there is a practical difference in strength between various commonly used epoxies as long as they are fully cured. There are probably greater variations in wood and construction than one epoxy having a greater break point at "x" psi or whatever the appropriate units are.
I wonder if the brittleness mentioned is a simple consequence of a faster curing epoxy being able to chip in a couple days whereas a slow epoxy might take 5+ days in cool weaather. My backyard experience jives with this but still given the thin layers and variety of woods I don't think there's a practical difference. You can use a fast epoxy for some things, slow for others. Check out Vaclav's website for more fun and reading http://www.oneoceankayaks.com/Epoxtest.htm