For any army that can take Sisters of Battle as allies, I think it is hard to look past taking Saint Celestine. She is hands down the best objective contester in the entire game and is super cheap and pretty hard too. Seeing as though 5 out of 6 missions now are objectives now, she has gotten even better.
Sure you have to take a Battle Sister squad, but they are ok and dont cost too many points. Plus you get the chance to throw in a lovely Exorcist too which despite the hit that all tanks have taken with 6th, is still a pretty nice option.
Dont forget all shots go to closest model. She makes a fantastic bullet magnet.