Use the higher of the two. No modifiers other than the PC's regular modifiers (including bonuses for items, spells, etc. I believe {
PHBp.136 - Saving Throws}). ~D
The interesting part is that, based on the description in the MM,
the weapon will escape damage altogether if you make your
Reflex save. Furthermore, if you don't, your weapon is toast
regardless of acid damage done.
Monster Manual p.145-146:
Acid:...The ooze's acidic touch deals 40 points of damage per round to wood or metal objects.
...A metal or wooden weapon that strikes a gray ooze also dissolves immediately unless it succeeds at a Reflex save (DC19).
If the Ooze did 40 points to a Heavy Mace (H:10/hp:25) and the
following rule is applied:
PHB p.135:
Damage to Objects:
...Energy Attacks: Objects take half damage from acid, fire, and lightning attacks. Divide damage by 2 before applying the hardness.
Theoretically, the mace would survive. But, such is life.
~D