Get the one for the area you need the most work on! And if you're just starting your GMAT prep, I'd recommend doing some general review first so you can figure out which specific book you'd benefit from the most. (That's what I'm doing, I'm starting with Kaplan Premier 2011.) Also, this way you won't waste those CATs before you've done some substantial studying.

Agree with the post above. Entirely depends on what your weak areas are.

BTW MGMAT SC is the best SC guide out there. It's really good to practice as it lists OG question by topic area. After you finish a chapter (i.e. topic area) you can practice by doing OG question on that topic area. I highly recommend it!

I agree with the others. Sign up and take the free test first (it's one of the six). It breaks down your strengths and weaknesses by question type, so you can have a better idea.

As far as which book to buy, I've bought Number Properties, Geometry, Word Translations and Sentence Correction. Of the four, I think all but Geometry are incredibly, incredibly helpful. Geometry was the least helpful only because it turns out I had remembered most of that topic, anyway. But if you're rusty, it'll help you out plenty, too.

Looking back, I regret not buying the whole bundle at once, because I'll probably wind up buying them all anyway.

I've bought all of them but I wouldn't really reccommend that unless you really think that you will use all of them. However, it really does depend on your weak areas. Number Properties has helped me the most (but I have only read number properties and the geometry one so far). I may eventually look at SC but I tend to do well on SC so its probably going to be one of the last ones that I look at.

I'm using all MGMAT & OG materials. Buy the whole lot of them, trust me. You'll finish 1 or 2 and then attempt some OG questions and realize you should've just gone through their entire system first. Then you're stuck until the new books arrive. Just buy all 8 guides and go through them even if you think you are strong in that particular area. It will not be a waste of your time.

I disagree. I bought all of them, and regret it because I don't need them to get the score I want. If your mainly interested in them for the CATs, which is sound like you are, then you don't really need all of them.
Like I said before, number properties was great so I would recomend that one. In my opinion, unless you are really looking for a 700+ score, you could get a good score with the official guide(s), number properties, official cats, and the mgmat cats just fine.
If your like me- not looking for an amazing score and or don't have enough time/motivation in the day to read 10+ books like most people on this board, then you don't need all of them.