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I usually take both. Or whichever one comes up first. But if I have to choose, I'll go with light against large dark armies (resurrection & holy word). And against light, I'll use darkness, with their unholy word.
But in general, I prefer chaos; light and darkness aren't my favorite skills.
Whatever comes up first can be exchanged for 2500 at the mentor later. But in general, it's better if the hero knows both spells and has a high magic skill. By the way, you can also play as mages, especially if they've learned the arcane sight skill.