I'm a mage, and my style of playing is using a bound sword in one hand and an illusion spell in another (with a flame atronach), occasionally playing stealthily when brute force just isn't enough.
So with that style, I'm torn between which defensive skill to focus on. I want light armor, so Alteration seems to work well there, but Restoration can work well with wards and fast healing. Then there's alchemy which seems to be the best to use, but requires the most effort and money. I just can't decide.