It is normally best to kill a single Pain Titan and then it's resultantant Madness titans. As the team will generally take more damage from two Madness Titans compared with the single pain titan. (A notable exception being if the party has a Life Bonder in it).
Pain Titans spawned from slain Titan Abominations will not droploot, but will yield experience and, with the appropriate bounty, Lightbringer points.