So I am wondering,

What is better for a mage - enchanting my gear with extra magicka or enchanting it with the "[school of magic] costs % less to cast" enchantments?

Cost reduction

You can essentially reduce your magicka cost to 0 with the right enchantment perks

This sells it for me! lol

Cost reduction 100% svcks the fun out of the game and is extreamly inorganic way to play. It looks like with the large amount of mana it takes to cast high level spells the developers intended for you to use cost reduction gear but the presence of cost reducing perks + the fact that destruction isn't designed to be a stand alone magic skill shows that they didn't really mean for you to cast for free.

I find the middle path to be much more rewarding for a mage. Focus on mana during leveling plus mana boosting enchantments plus cost reduction perks/gear to get between 50-75% off in 3 to 5 schools of magic. That way your actually using your magic abilities all game and leveling them organically instead of grinding enchant. Destruction may seem gimped when you use it by it self but its a hell of a lot more effective when your enemies are fighting your summons and each other while you blast away in the back.

If you plan on using Destruction as primary damage, then you should go for at least 80% mana reduction.
If you prefer to use Frenzy or summoned beasties, then you can go with a lot less cost reduction or a big mana pool instead.