You choose DirectX 10 or DirectX 9 ?

Simply put, DirectX® is a Windows technology that enables higher performance in graphics and sound when you're playing games or watching video on your PC.
At the core of DirectX® are its application programming interfaces, or APIs. The APIs act as a kind of bridge for the hardware and the software to "talk" to each other. The DirectX® APIs gives multimedia applications access to the advanced features of high-performance hardware such as three-dimensional (3-D) graphics acceleration chips and sound cards. They control low- level functions, including two- dimensional (2-D) graphics acceleration; support for input devices such as joysticks, keyboards, and mice; and control of sound mixing and sound output.
Because of DirectX®, what you experience with your computer is better 3-D graphics and immersive music and audio effects.

DirectX®10 Benefits
DirectX®10 offers a wide variety of benefits to gamers. Let's dive into the details now:
• More Life-like materials and characters

              

                          DirectX 10 in crysis

                    

                             DirectX 9 in Halo

You can see the difference for yourself when we compare Picture 2, a screenshot taken from 'Halo: Combat evolved', a PC game which was among the best of its class at release to Picture 1, an image from 'Crysis', a PC game which is slated to release soon. Halo uses DirectX®9.0b to support its graphics while Crysis makes use of DirectX®10.
DX10 adds a new level of realism to games by making characters more life-like. You can see the incredible detail in the screenshot above. The facial expressions, details of the face, the handle-bar moustache and the glazed looking eyes all add to the realism.