If you had to choose only one that you could buy, which book would it be and why?

That depends.

How comfortable are you with creating statlines for enemies yourself? If you're very comfortable with it, Disciples is better than Creatures due to sheer amount of fluff and ideas.

How much do you like Chaos and Heretics as adversaries? If the answer is "very much so", Disciples is great. If the answer is "not so much", Creatures is better as it contains a wider variety of enemies.

Other than that, both are great books, and I can't really choose.

Disciples. One of the best RPG books EVER!

As the previous posters have said it very much depends on what you want/need for you game and your group's style of play.

CA is very much a "monster manual" - there are some great ideas and plot hooks there but it's mostly a book of stats.

DotDG is a setting book (looking at the various notable threats to the sector) with NPC stats including.

On the whole I've probably got more value out of DotDG and would rate it as a "better" book - but that's in part because it's a better fit to what I've tried to do with my campaigns.

I am sorry to disagree, but

TO HELL WITH "IT DEPENDS ON"

(sorry, Nerd King! Nothing personal )

DotdG is clearly the better buy. It is much richer in background AND you get some stats for this and that as well.

I think i'll give them a miss for now. I picked up Daemon Hunter and I might get Blood, and we'lll see how it goes. If they were free i'd happily own the lot, but I can make up my own dark disciples and while CA has some good content I think only half of it would be practical for me.

DotDG is great; it's impossible to so much as skim through it without coming up with ideas for lots of cool scenarios. CA , on the other hand, has been much less useful than I thought it would be. It's not a true "Monster Manual" with tons of creatures covering every power level- very few of the critters therein are appropriate for low-level parties, and most are way too specific to be used as random encounters. The definitive DH "Monster Manual" has yet to be written...

I think I agree with your view on CA. The ideas, themselves, look good, but as a whole I'm not sure it's going to give me what I need. I'm not really after weird critters or death planets and I don' tplan on using Orks or Eldar - at least as straight adversaries. Eldar are more than that.

DotDG has by far been the most inspiration to me, but other than that, I've basically not used any of the stat blocks from it.

CA, however, has been very helpful for quick plot hooks when I am stuck as well as for things like Orks and the like.

I would say that for an average game with no specific enemy, CA is definitely the way to go. If, however, you're doing an investigative/cultist game then DotDG is the way to go (as it is full of all 3 categories of nasties).