Target negative resist VS Plus Spell Power

[TheLurker]

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

[DacianDraco]

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.