This toon is going to be an illusion mage/assassin. I can't decide where to use Light armor or magic robes.

Light armor is good because I can get full use of the DBH armor (which I'm going to be doing) and I won't have to use alteration

But then I've wasted perks in Light armor because there isn't any good armor with Illusion enchantments (Robes of illusion)

I want to use the robes for the illusion but then I won't get any armor bonuses for light armor, which is what the main armor I want to wear is.

Can anyone help me decide?

Sounds similar to my thief/rogue. With sneak and illusion you hardly take any hits, so alteration and light armour level up very slowly - especially when you don't use flesh spells because you don't have quiet casting early in the game. I ended up putting a minimum amount of perks into those.

Nightingale armor would be my choice.

I'm not great at making assasins.. But as you're investing in illusion/sneak I'd go robes.
DB boots muffle don't they? If so wear them also.

You won't be in combat really so armour won't really be needed I'm guessing.

The nightingale armor maybe? I believe the nightingale hood has an illusion bonus.
I might be mistaking though, i just started a dark elf assasin. And she is going for nightingale armor.