Go with Illusion - it's brilliant fun, if used in conjunction with Sneak you can hit any enemy in the game with dual-wielded Fury (a novice-level spell) to make them kill each other. You don't even have to get your hands dirty. It's also very easy to raise, and the Aspect of Terror perk which adds 10 levels to your Illusion spells also adds 10 damage to some fire spells - couple that with the Augmented Flames perks in Destruction and the Novice-level Flames spell does upwards of 20 DPS, or upwards of 40 dual-wielded (but not overcharged as that costs unnecessary extra magicka), which is strong even at level 30ish. It's arguably not worth concentrating too much on Destruction though, because whatever happens it's weak by high level (50).
I'm rolling an Illusion/Conjuration/Sneak mage at the moment. It's extremely fun and extremely powerful - you can control entire battles without taking a scratch if you use your spells properly. You have to use a load of different spells and keep changing them on the fly, but there's a lot of potential and lots of different possible approaches.
My Dark Elf Ill/Con/Sne mage regularly takes out entire forts with the following process:
1) Cast Muffle [EDIT: and Oakflesh (or higher Flesh spell) if you want to raise Alteration too, but this isn't necessary]
2) Sneak to a decent vantage point where you can see an enemy at a decent range. Stay hidden.
3) Dual-cast-overcharge Fury (dual-casting Illusion spells more-than-doubles the max level the spell affects). Watch as that enemy attacks his friends, killing them or being killed themselves.
4) Repeat 3), or go straight to 5)
5) Finish off any stragglers with Bound Bow Sneak attacks.
6) Reanimate the most powerful corpse to fight by your side.
7) Find the "boss" of the area. Sneak Bound Bow attack them to get their attention; let your undead thrall tank them.
8) When your thrall is dead, dual-cast Calm on the boss. They will stop attacking you.
9) Sneak again, conjure a Bound Sword, assassinate them from behind.
10) Reanimate the boss as your new thrall

Of course this is just one option - the simpler (although less fun) way to do things is just to conjure an Atronach (Frost ones work very well as they're melee fighters) and let them take the flak while you ping arrows in from a safe distance. I prefer the more complex option though, because it leaves you totally in control of fights with several enemies.