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I choose based on which Castle I'm playing as... unfortunately, it's not always possible to choose. If I'm playing as Dungeon, I need less Knowledge – Mana Vortex gives a certain advantage...
If the hero is a fighter, mana is important – strong combat spells aren't really necessary, but frequent use of weak but massive and important spells for the fighter (slow, bless, etc.) is a good help in battle....
On average, I prefer Knowledge!
If I have to play as Solmyr, I definitely choose spell power!