Ebonyflesh (expert) gives you 100 armor rating, 300 with the perk. For a duration of 60 seconds, 90 seconds with the +50% duration perk and over 3 mins dual casted with the duration perk. You can also add the lord stone to this for a total of 350 armor rating.
Dragonskin (master) gives you 80% damage resistance (the maximum) for 30 seconds, 45 seconds with the duration perk. But if you dual cast it (likely a bug, master spells aren't normally affected by dual casting) then you will get 99+ seconds of 80% damage resistance, which IMO is more than enough to kill the people you need to.
You can get the unique archmage robes which give you 15% spell reduction for all magic schools, +50 magicka and +100% magicka regen rate. If your enchanting is at the max, then you can enchant your own clothes for better enchantments than the archmage robes, can be increased further by exploiting alchemy.
So really you are better of going with armor as the unique enchantments on the unique robes can be beaten by your own enchantments. But I think its more fun going with alteration + clothes for a mage, so thats what I do for my sneak mage ha.