Librarian vs. Chaplain

I think *neither* is tactically flexible. Both are dedicated close combat units in the end, really-- they both come with fancy close combat weapons, and in both cases, their abilities are best used in close combat-- in the Librarian's case, a force weapon and psychic powers that have very short range or have some effect on how combat flows; and in the Chaplain's, Fearless and re-rolling misses on the charge. The tactically flexible HQ choice is the Commander, since he can be equipped with ranged weapons and buffs the entire army's leadership, which is quite a bit more useful when you have a gun line than having a model that can make close combat rather one-sided.