As the game is pretty bad for calculating the damage, any +% to damage is added to the initial damage skill you have. it is additive but take in consideration the amount of projectile you have and that scales a lot better : L/CL vs lightning fury(java)
For lightningSorc and fireSorc - %resistance is better then + %damage
For FrostSorc +%damage is better then - %resistance(when capped at -135 % including mastery)