I have been bobbing back and forth between Fel Armor and Demon Armor.
Demon Armor gives roughly 10% more physical damage reduction and 20% bonus healing through effects and spells.
But the HP5 of Fel Armor and the bonus spell power combined with Soul Leech seems to out-pace anything generated through the bonus healing that Demon Armor could generate.
Maybe Demon Armor is more worth it in something like say - Affliction - where Drain Life is part of your bread and butter, not to mention all the passive healings you gain from talents like Siphon Life, Soul Siphon, Shadow Embrace, and Haunt's effects.
Perhaps I should stick with Fel Armor for Destruction.