Radeon HD7950 gets my vote
You can find some great models for around $310.
Difference between the two brands would be performance and features.
Geforce GTX offers a few things that Radeon HD does not. A few of them would be Adaptive V-Sync, CUDA, Nvidia 3D Vision, and PhysX
Radeon HD offers 3D game support as well, but it's pretty confusing to configure so I wouldn't consider it a selling point.