Right, as i deleted my lvl 49 jack-of-all-trades character, i need to start again, so i was thinking pure mage, and theres 2 thinks i need to know, which kind of magic, and is there any major disadvantages to this?

I used lighting exclusively, its good against mages but its more expensive than say fire or ice per cast.
Take that how you will.

I want your avatar picture.

Now, onto the Magic. I find using all three has it's advantages. They are each useful for different things, lightning drains Magikca, Frost drains Stamina. I haven't noticed the effect that much as I'm not sure on how much Mana enemies have in the higher levels (It seems like a lot)

I mainly stick with Fire, simply because I enjoy blowing things up in big, fiery explosions. Luckily, you can try out each Destruction subsection without investing any perks into all of them, play around with each of them and see which one you favour.