My favorite part about the Vancian magic system is that it keeps the ritual aspect of magic intact through the preparation phase. No, magic is NOT something "Within" someone that merely requires force of will to use like some sort of superpower. It's a power greater than you that must be appeased and invoked. It's part of the world, not the caster. Unfortunately, D&D glosses over all the interesting parts of magic by relegating it to a simple one-hour prep phase and spell component pouch. There's no mystique or ritual to magic. I'd have prefered "A spell takes X amount of time to prepare, and Y sympathetic ingredients, and an envrionment of Z", with the ability to either hold onto the spell Vancian-style (With spell-slots restricting the number of spells you can prepare), possibly risking losing some of its potency, or letting the spell go off immediately as a ritual. No "Must rest 8 hours to clear mind" of "Cannot regain a slot used in the last 8 hours" garbage. Each spell would have its own preparation system. Some would be short and quick (Good for blasts), others might have extensive preparation times.

Well, the above is for Wizard casting. Wizards and Druids are the redundant class (And the same source material nature shows in D&D!) not Wizards and Priests.

Priests are different, because they have a powerful patron (Such as a God, Demon Prince, Dragon, or Sorcerer) that can channel their power through them instead. The more powerful the priest, the stronger the bond between the two.

So(u)rcerers are essentially living gods, being a living font/battery of power specific to the sourcerer. A powerful sourcerer could gain priests, fit to become a God-King as its own power grows and expands.

Warlocks/binders are like anti-priests, kind of, and are usually Wizards or Sourcerers as well (Or badass warriors). They have a little bit of power from their sourcery or wizardry/druidism (Or even badassery), that use the power they have to harness lesser occult spirits into serving them, and becoming more powerful through that - think like Belkar using the Eye of Fear and Flame: Belkar alone cannot shoot fireballs or cause creatures to run in irrational terror from him (Though most run in rational terror because of his skill at slaughter), but he's able to dominate the skull into serving him. Imagine if he had an even more forceful personality, and found other skulls/intelligent objects/spirits with different SLAs at their disposal - he would be able to essentially become a tyrant caster through using his ability to scare the skulls individually into serving him, and collectively using them to keep each other in check to avoid his wrath. A wizard could instead use rituals and deals of binding as old as the earth and spirits themselves to gain the loyalty of these spirits/skulls/magic items, and a sorcerer could bind with his power)

As for the balance between them - A wizard with the right know-how and a strong enough mind and stomach for the rituals has the entire multiverse at its beck and call by pulling on the powers that draw upon them. A priest is limited in terms of the power of its patron and its connection - but the height of the connection is "We walk as one", and there's always a bigger patron (And, it's possible to assist the patron in becoming more powerful). A sourcerer's power is limited by the development of his powers - a low-level sourcerer can do a few weak things on its own. A low-level wizard knows only a few rituals that achieve minor tasks on the world. A low-level priest can use only a fraction of it's patron's power. A low-level warlock can only bind a few spirits to its bidding.