Wiki data table http://en.wikipedia.org/wiki/Compari...#FireGL_series

I lack knowledge to decide, which one of those two cards will be better for my needs. I intend to use it for gaming with open drivers. I made my mind to game on this and nothing will change it.
Basically when I can't hit high FPS, I set lower resolution before I turn details down. So I will prefer a GPU that lets me play full-detail low-resolution rather than low-detail high-resolution. Of those two, which one fits my needs better? Which parts of hardware are important for me?