Anytime you increase resistance against an element, you wind up taking less damage. How much so is really the question, and one I'm not entirely sure of.
VX was simple in that damage was determined by A-F checks:
A = 200% damage taken
B = 150%
C = 100%
D = 50%
E = 0%
F = Absorb
F meant if you were hit with that element you were healed instead of damaged.
With Ace, you can set your own formulas up, and I'm inclined to think the greater your resistance the great damage you can sustain, including the possibility of absorbing the element. However, I don't know if that means your resistance would increase by 125% base when the item is equipped, so if you were 50% resistant, you're not 175% resistant, healing you for 75% of the damage, or if that 125% is added and divided into the base stats, so at 50% plus an additional 125%, that would make your total at 87.5% resistant.