I guess I need to do the calculations myself? Likely too many factors to consider, might be different for different builds/enemies.
I tried to do the calculations doing the maxroll gg d2 planner. My finding: The -res didn't make any difference dps wise on decreped or amped monsters. So I put in a bug report on the maxroll team. They said they use game files to base the calc on, so logn story short: There might be a bug where -physical damage on enemy is not even taken into account. So playing a frenzy barb myself I would rather go with the ed, than the -res.
Also: The calculation depends on a number of factors: How much ed you got already. If you use amp or decrep on mobs, which was already pointed out. So it's not a simple this is better than that question.
With amp, ED should be better
Without, -10 res should be better
I would say the exact same as a rule of thumb. Echoing Strikes Warlocks with a fire / light res dmg aura + curse hephasto don't need -res, they would go with ed.