Well, CS3 is more classic. It doesn't have a collection of new, confusing tools, or new tweens. It's easier to use, and has a slower, less difficult learning curve.
CS4 is more powerful. It has new tweens that can handle multiple objects, and powerful new tools like the 3D tool and the inverse kinematics tool, which you can use to make very professional-looking animations. However, it has a much steeper, much more difficult learning curve.
With the right experience, you can make anything equally as good in either.
I recommend you start with a trial of both and see how it goes.
Whichever you get more done with in 30 days should be the one that you buy.
CS4 Trial
CS3 Trial