If you're playing BA or an assault-oriented army, chaplains are better. If you're playing a more shooty army, I prefer librarians. If you don't put too much stuff on them librarians can be very reasonable. I prefer an epistolary with a bolt pistol, iron halo, veil of time as his first power, and fury of the ancients as his second power. That comes out to 162 points and has the ability to throw lots of hits and pinning checks at your enemy even if they're in cover and is still a certified badass in cc with veil of time (giving him the equivalent of almost a 2++ save and a better than 2+ save as well as rerolling hits and wounds). Not to mention that psychic hood kicks enemy psykers in the balls. Sure he'd be stronger with an adamantine mantle and terminator honors, but that's 50 points you could spend elsewhere.
Nothing beats a chappy for leading assault squads though.