Well, if I'm Player A, and my EH has F7 and Epic Strike and I declare a Duel against Player B. His EH has F6 and Epic Strike. The odds are already in my favor and I'd rather not burn the Might, just being happy taking a chance with the normal roll. But if I knew my opponent was going to call Epic Strike then I would as well as a counter.
To be sure I'm reading your interpretation correctly the phase would go something like this.
Player A has Priority and declares Duel
Player A now has the option to declare Epic Strike (he has priority and goes first)...but for reasons given above decides not to.
Player B declares Epic Strike
Player A is SOL
The interesting thing behind this is it would mean if you have a Hero with Epic Strike you almost never want to be the one to declare the duel against another ES Hero as you're better off reacting (and possibly catching them off guard). Likewise, if you ever do declare a duel against another ES Hero then you must declare Epic Strike as well for the same reason that if you don't they probably will (hence prisoner's dilemma).
And if you were Player B, would you remind your opponent about the option by questioning them ("were you going to call an Epic Strike?") or just sit quiet until their part of the turn is over and then spring it on them. If the later and they say "Oh yeah...I forgot to do that too"???
This is where I was thinking it might be best to have a hidden indicator that you each select if you're going to do Epic Strike and then reveal. Avoids possible hard feelings and keeps things level.