Elemental Force procs for more damage than you'll get from Sunfire being added to your spells. Last I checked, Sunfire scales really poorly, like +10 fire/arcane damage @ level 29. You'd be better off with +30 Spellpower to weapon enchant (+16 @ 29) if you were using a static spellpower enchant. However, with the way your main spells benefit from spellpower, +16 spellpower enchant would add anywhere from 22.88 to 36.32 damage per attack with Incinerate, Conflagrate, and Chaos Bolt. Elemental Force does not scale with spellpower, does not require any other stat to do it's job, and procs for 110-220 damage at a rate of 10 rppm. That's approximately one proc every 6 seconds, which would substantiantly outweigh the damage gained by a static spellpower enchant.