Definitely experience points for finishing quests.
Currently having essence doesn't make you a better caster. It just makes you have a higher mana pool. And you can reduce that mana pool by using specialty spells.
If you want to have a better mage you have to increase your Intelligence and Wisdom.
However, currently if you want to win the game magically (assuming Spell of Making still costs at least 20 mana) you have to use level-ups to buy essence.
This is under the game standard that, like Warcraft III, leveling up is a Necessity to winning the game.
All this talk about High Level Sovereigns ... or even a Champion you imbue. Honestly low level Sovereigns won't have a chance. you can talk about Land Grab vs Small Empire all day ... but you need a high level character to win. (at least for Magic Victory and Quest Victory)
Now that that is out of the way ... with this option, Intelligence is the obvious choice to become a better caster. It (indirectly) increases your mana pool, and it increases the damage of spells.
If Intelligence becomes all that ... then perhaps Wisdom should become a bit more useful as well, although it did at one time dictate Spell Point Cap and still dictates Mana Regen.
Of course, an Alternate could be +0.5 essence per 10 intelligence and +0.5 essence per 10 Wisdom (on level up) ...