Leana Anita
UA will always outdps ruin unless you stack ~45% crit (with capped hit), which is doable with full raid buffs, an elemental shaman, and a moonkin, but isn't practical and requires a gear level that probably won't be obtained by someone asking for spec help.
Well, every warlock in my previous guild was contagion/ruin with exception of 1 who was demonology for Kael and Leo, and the warlocks were the primary dps on raids. Dont think they all had a moonkin, or an ele shammy. In fact, we didnt have any ele shammys in guild cause the raid leader didnt think they'd be worth it, and we sure as hell didnt have a moonkin. :p
I dont really have numbers in front of me, just assumed they knew what they were talking about. Even if it isnt better dps in the long run, bigger crits are loads more fun than putting up an extra dot.