I have used both Nvidia and ATI and can tell you that there's no real difference. Look at it like Ford and Chevy... different cars, but both get you to the same place in the same way.
ATI has been slamming Nvidia in the price/performance area for a while now. What dirkpitt said is true, ATI used to have a real problem with drivers but that seems to now be a thing of the past. And the 5xxx series can fully utilize DX11 as long as you have Win7 and the game utilizes it (obviously). With what we are seeing so far regarding Nvidia's upcoming cards I have no problem recommending ATI's 5xxx series.
Pick whatever will fit in your budget, but make sure you have the PSU that can power it.
ATI has been slamming Nvidia in the price/performance area for a while now. What dirkpitt said is true, ATI used to have a real problem with drivers but that seems to now be a thing of the past. And the 5xxx series can fully utilize DX11 as long as you have Win7 and the game utilizes it (obviously). With what we are seeing so far regarding Nvidia's upcoming cards I have no problem recommending ATI's 5xxx series.
Pick whatever will fit in your budget, but make sure you have the PSU that can power it.