ATI is usually the cheaper option of the two. Which isn't half bad, if you look at their POWERFUL 4800 series.
Nvidia is usually on top of drivers and offer very good quality when it comes to their video cards. While ATI usually has driver issues after a new card has been released.
A LOT of games usually have the logo "Meant to be played with Nvidia." So I m guessing Nvidia has more money to spend when it comes to promoting their products.