adampcman :
ATI is cheaper - Nvidia is pricier.
ATI drivers have better chance to bug games - Nvidia drivers have less chance to bug games.
ATI have "frame drop" - Nvidia have more stable fps.
But that doesn't make ATI a bad choice or Nvidia a better one. They both have their good and bad sides. That's why there are so many ads to convince us what to buy...
The driver situation not so cut and dry. ATI drivers have more small glitches generally, but there are almost always a good set without the glitch if you try some of the older ones. Nvidia drivers cause more hard lock ups and game crashes (proven by Steam surveys).
I'm not sure what you mean by ATI having more "frame drops". If you look at the benchmarks, the new 6000 series at least, the Nvidia cards usually have the lower min FPS, but slightly higher average FPS.
What I can say that is good about Nvidia, from my experience, is they are less likely to have hardware issues, which I believe is due to them clocking them at a safer level to prevent issues.
For me, when it comes to choosing a card from either brand, it comes down to PhysX vs SSAA and MLAA. I currently use both due to having a spare card to run as a physx card.
If you play games that use physX effectly, and you wish to use that ability, go for Nvidia.
If you find aliasing (jaggies) annoying you in games, ATI has better options to get rid of them. SSAA is officially supported, and can be useful in games that aren't overly demanding (I use it in Dragon Age Origins and love it there). I also play a few other games which AA isn't supported and can't be forced on (Risen for example), MLAA is post process and can be forced on any game.
I've found a new use for MLAA now that I'm playing Two Worlds 2. Two Worlds 1 & 2, with aliasing on, causes a glowing outline around objects and characters when you turn on AA. It's almost worse than the original aliasing. With MLAA, this problem doesn't occur. In Two Worlds 1, I use both MLAA and MSAA for best results.