If you drain tank, I'd go with Demon Armor since you take less damage and heal more through Drain Life and Siphon Life.
Being a Destruction Warlock myself, I tend to use Fel Armor more, because things die before they can reach me (Immolate, Conflagrate, Chaos Bolt, dead) and I use Life Tap instead of wasting time drinking/eating. Thanks to Soul Leech and Fel Armor's health regen, I can keep up the killing with zero downtime (tested this by going on a killing spree in Borean Tundra for about 45 minutes, killing maybe 100 mobs and not having to drink once).
I tend to use Demon Armor when I PvP with Affliction because it makes me last a bit longer. Additional mitigation + additional health gain from heals + Siphon Life = sweet. Now I'm level 73 and most of the time if I get ambushed in Battlegrounds, I'm dead anyway (79 Rogues, yay), so I just go all out of the damage and rarely use Affliction in the first place. If I can't burst it down before it gets to me, I'm usually dead anyway.
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