Elemental advantage: +45% to elemental bracket for you, -25% to elemental bracket for enemy (which as Slashley said, is basically 25% less dmg received, most of the time BUT it's not damage damage cut).

Light/dark natural advantage: +3% to elemental bracket, regardless of opposing element. If they're up against each other, player side will receive every perks of elemental advantage + still the +3% bonus to elemental bracket (it only adds to your damage, no effect on damage received).

Elemental advantage on burst: burst streak gets a x1.45 multiplier. Individual bursts are calculated like: normal damage x (natural burst multiplier+burst buff). Normal damage includes elemental advantage already, natural burst multiplier is decided by rarity, burst buff includes exceed weapons, and active/passive buffs in battle.
This means that only the burst streak (kinda indirectly) receives elemental advantage bonus twice.

While not omnipotent, elemental advantage has quite some nice perks. A bit difference in stats alone won't make your best element worth bringing over advantage team. Shit setup of advantage vs well-covered team of best element might, tho.

P.S.: we shouldn't forget about better debuffing chances for advantage & better resistance to debuffs from enemy.