Diablo 1 HD Hellfire Mod | Awake

Which of these two attributes provides a greater effect?

For example if I have one staff that has +30% Spell Power and another that has Target Lightning (or any other element) Resistance -5% how should I decide which to use?

Question coming from a lvl 38 Sorc having issues finishing Nightmare Terror Domain

Get both, staves have less affix pool than other weapons so it's not hard. And Spell Power is better if target is not immune, because spell power comes in higher % value and affects all spells regardless of element. But later on you will encounter some monsters that are immune to all elements, so -% Target (element) Resistance is a must, otherwise you do no dmg even with whatever % Spell Power you have.