First, "Elemental Shields" is an Open list whereas "Dispelling Ways" is a Closed list...
Second, whilst most elemental attack spells come from Essence spells, not all do, such as Shock Bolt in "Light's Way", which is an Open Channeling spell. A such, a Cancel Essence or Dispel Essence wouldn't work against it...
Third, cancelling or dispelling spells give the caster a chance to avoid all effects from a spell whereas an "Elemental Shields" spell gives him the certainty to be partially protected from an elemental spell. In other words, if the opposed party's spellcaster's level is way above the caster's level, he'd better off with "Elemental Shields" since he wouldn't be able to cancel/dispell most of the opposed party's spells anyway...