The main reason to use epoxy for high stress structural areas like wing saddles, firewalls, landing gear blocks, and wing mounts is the thickness of those joints. Any glue that requires exposure to air to harden isn't ever going to harden fully inside of a wing joint. With epoxy, you get 100% hardening throughout the joint assuming you mix it right. The second reason is the properties of the cured epoxy itself. Cured epoxy is basically a flexible plastic. It can be pulled on and vibrated without breaking. If you don't believe me, spread a dab of whichever glues you have available on pieces of wax paper and let them cure. Cured CA and poly glue will break like glass; no strength at all. Aliphatic will too. But the Epoxy will actually take some effort to break; some of them will just bend back and forth and not break at all. The other glues are great for their intended purposes and will make joints stronger than the surrounding wood. But epoxy is the perfect glue for these structural joints.

As for the original question, the biggest difference will be the soaking in. So 30 minute epoxy is the way to go for anything structural. I don't think there is any benefit to going with longer working time epoxies since properly mixed and applied 30 minute epoxy will make a joint so strong the surrounding wood will break before the joint pops loose in a crash anyway. And as stated above, 30 minutes is the working time, not the full cure time. It's the best policy to wait 24 hours before flying a plane after gluing it. Don't even work on it or move it around during that time if you can help it, because the joint is fairly fragile for those first few hours of curing.