Originally Posted by Bobicon
If you are planning on OCing it at least more than 15-20% over stock speeds I would suggest a 6970 because of the different RAM (speeds or timings, not sure which one it is) used in the 6950 and 6970. So you will be able to get a more stable OC with a 6970.
The ram chips on the 6950-6970 are different in both voltage and timing, but I've had 3D benches running on my 6950 for 10+ hours with no problem at 950/1325 with 1536 shaders.
Additionally, it was proved by numerous respectable sites that, upon unlocking all the shaders, the 6950 performed near enough identically to the 6970 with the same clocks.
If you want an easy route to more reliable performance, though, you're better going for a 6970.