Stamina first, then Intelligence.
A heavy DoT ******* means never having to say your sorry. It also means having to outlast your opponents. Since cloth is the only armor we'll ever wear, more HP's are your friend, and more HP's = more Stamina. I've heard of Undead Warlocks with the right talent builds surpassing 6k HP by cap.
Since we'll never have the nuking power of a mage Intelligence takes a back seat to Stamina. This is the number 2 stat to improve, but will always take a back seat to Stamina, just because of the Warlocks fight style.