Both have their advantages and disadvantages, so you can't really say one is better than the other.
-Tends to focus on price/performance.
-Currently has released their generation of cards first and in larger amount than nVidia.
-Drivers tend to be flawed and difficult to work with.
-Holds the performance crown for fastest single GPU card (in gaming)
-Drivers tend to be matured
-You may have to pay a premium price for certain cards (i.e. When the GTX 580 was around $500, the GTX 570 only cost $350 and their performance difference was about 10 to 15%).