So i decided to create a stealth/magic character. So far i have...

Alchemy
Marksman
Illusion
Light Armor
Sneak
Security

but i cant decide if I want Conjuration or Mysticism. I like conjuration because it will allow me to summon Undead which is a good addition to the theme of my class, but mysticism has good spells too. Anyonen have any suggestions?

I'd go with Conjuration.

Both could be useful. Conjuration could be useful if you get detected and you're outmatched, while Mysticism's spells would compliment your character. I would go with Mysticism personally, but really it's what you deem more suitable for the RP.