Check with the office. For a while, you could get the books for free (but you have to pay for shipping) by proving that you'd bought the 2007 books. I'm not sure whether the promotion is still going on, but if you still have your receipt and can send that in, then maybe you can get the new ones for only the shipping cost. Send an email to
studentservices@manhattangmat.com to ask.
There are some decent changes in each of the books, but there are also large portions that are very similar. In general, base your decision on where you're struggling. If you're already doing very well with SC, don't bother getting the new book. If you're really struggling with CR, it may be worth getting the new book. That sort of thing.
The math books have more practice problems in them (the in-action problems at the ends of the chapters). They also contain more material for advanced test-takers - things that people would see at the 700+ level. If either of those things is relevant to you, you may want to consider those. If you only want to get some, number properties is most important, followed by equations, inequalities, and VICs. ("Important" means a combination of the magnitude of the changes and the frequency with which these things are tested on the exam.)
There are some more substantial changes in the verbal books than the quant (in terms of the fundamental techniques and grammar explanations). In SC, there's a lot more detail (though some people have told me they think there's too much detail now!), and in RC and CR, the techniques have been revised / refined fairly extensively.