For phys, at least with Echoing Strike or another build that is reliably cursing with amp damage, base mobs with 0 physical resistance will be at -100 after the curse and you won't benefit from the extra -res from the renewed sunder. And for immunes, sunder drops to 95 res, amp damage to -5 res, the -res on the sunder up to -15%, which is a ~9.5% bonus. That will beat out ED if you have a lot of off-weapon ED already, but if you are low on off-weapon ED then the +100% ED would be better. The breakpoint is probably around 900% ED but it depends on the actual rolls of the charm. So it depends if you want to prioritize immunes or non-immunes, and how much offweapon ED the build has.