I'm debating on taking either a chaplain or a librarian in my army. I currently use a chaplain in terminator armour with my thunder hammer terminators, gives a nasty re-roll. Though after that the chaplain is just like a terminator with a power weapon and two wounds.
The librarian seems interesting, particularly with null zone which wouldn't allow invulnerable saves and with avenger or smite would add a little better shooting than the chaplain as mine takes a combi bolter.
What do you prefer, and why?