what i better to choose for rendering ati radeon 7970 or nvidia 680?
Too lazy to go look at card specs, but generally speaking, nVidia cards do better with OpenGL, something that is important to Blender.
Also nVidia's CUDA is better supported in the new Cycles renderer than OpenCL.
As always, your mileage may vary. Having a good OpenGL driver for your card is the key.