Lib Or Grand Master?

I'm restarting my grey knights force, to use alongside my larger Deathwing force, but i'm unsure which HQ to use (lib or Grand Master) with a Vindicare, Land Raider and strike unit, Bearing In Mind that the my Primary detachment will all be deep striking terminator units.
Cheers

it depends ...

generally speaking ... the libbie is a good HQ choice for small GK forces (1000 to 1500 points) and the GM for larger (>1500) but it depends a lot on your playing style and the way you're feeling yourself with one or the other. I prefer the GM for his special rules, the grenades he can carry and his CT / CC awesomeness, but some brothers choose the libbie for the buffs he gives to the army and his versatility.

it depends also on your HQ choice for the deathwing, if you chose a libbie for the deathwing then choose the GM for the GK, and vice versa...

Aubec

If you're taking the GK as the allied detachment then I'd suggest neither: go for an Inquisitor With such a small detachment you're missing out on the GM's best use which is Grand Strategy and the Librarians main strength of buffing units(since he can only affect units from the GK codex with them). Since GK IC's can't join the DA units I'd suggest the now popular terminator psyker psycannon inquisitor. he can join the strike squad and use his psycannon and Prescience to provide some mid range fire support for the deepstrikers.

I'd say libby is the way to go, for your force that is. You can always take advantage of his powers in your entire force, taking allies allows you to have enough scoring units in your force, so a GM is not that useful anymore.
Take the libby I say.

I also prefer the GM for his Grand Strategy, grenades, and psycannon.

To be honest I'm inclined to agree with Aubec, it really depends on the points and the way you like to play. Personally I like a gm but with a psychic buff. That said the smallest games I play are 1500 Pts and normally 2000 so for me it's a good solid point spend

If you need some of your force to be able to do other things (eg take objectives) then yes Grand Master is brilliant. Otherwise save yourself 25 points and take a Brother Captain.

Now, Grand Master vs Librarian:

Grand Master has more attacks, higher WS & Ballistic Skill, more wounds, and also cool things like melta bombs, blind grenades, rad and psychotroke. Meaning if you put him with a unit like Paladins they will be a total bitch to destroy. Oh he also has Iron Halo, so give him a sword and it's 3+ inv save in CC against AP2 or AP1 weapons.

Librarian on other hand as people have said is more of a support character.
Give him the Summoning and he can bring a dreadknight to your side from across the board. Put him with a unit that HAS to survive and give him The Shrouding and Sanctuary and that unit will be pretty unassailable.

Personally I think the Brother Captain/Grand Master is better if you want a combat oriented leader, and the librarian is better if you want a shooty army