We go for either. Some Warlocks will panic when you jump on them and Siphon Life both you and your partner. PoM heal through this with Renews/Shields/Casted Heals when you can. And its a pretty easy fight. If they are not doing this, get the Rogue to blow his Evasions, then jump back on the Lock. We usually just jump on whoever is not Undead and/or has worse gear or a poor spec. But our default is the Lock if the Rogue is Shadowstep.