Clarification
Just wanted to chime in with my 2 cents. Actually you should not be choosing between polish or wax, as the two are designed to be used in conjunction with one and other. Its also important that you purchase a good quality, I would recommend a product from
http://www.meguiars.com/ , they have been among the most highly recommended paint specialists and have extremely good products.
Polish:
The role of polish is to nourish the oils in the paint; and thus it is designed to penetrate through into the paint. This is where you will get your rich luster, and this adds a large degree of the silky smoothness that you will experience after you polish. I cannot emphasize this enough, this step is crucial in maintaining the appearance of your cars finish throughout the course of the cars lifetime. Most people don't realize that their paint needs to me maintained, and thus you see many old cars with dried out flaked or peeling paint.
Wax
Although the popular perception is that you wax a car to keep its paint healthy, and smooth, the role of wax is to protect the nourishing oils that came with your car and/or you have applied while using polish. While its true that the wax will make your car very smooth, without applying a good quality polish first you are only delaying the inevitable degradation of your paint.
I would highly recommend using these items in conjunction with one and other. Realizably in normal conditions you should only polish your car twice a year, and you should wax your car every few months. Also if when you apply a polish to your car, it is useful to first remove any built up layers of wax that might be on the car prior to application, this will allow for the polish to work most effectively. One of the most effective ways of doing this is to apply a small about of laundry detergent to the car, thus stripping any residual wax. Then apply the polish to the car, and finally the wax layer to protect it.
I highly recommend when washing your car not selecting a car wash that uses the liquid wax. These chemicals that may be great for your paint finish, can wreak havoc with any plastic or acrylic parts of your car, including causing fogging of the headlight lenses ect.
Lastly if you have a finish that seems to have dulled significantly, I would recommend using a fine level cutting cleaner from the same companies that manufacture wax/polish ect. This first step will remove a small amount of surface material, to expose the healthy underside and regain it original lustre and intensity.
~Tres