5870 is better unless you can unlock 6950 to 6970 

Not all 6950s are unlockable, only reference 6950 2GB VRAM models and like 2 non-reference (that I cant rmeember off the top of my head) can have their shaders unlocked. The card will then need to have its bios flashed either to 6970 (can be dangerous) or have their shaders unlocked (less dangers, requires overclocking manually to 6970 levels).

Depending on the price you can find either card for will depend on which is a better buy for the money (typically 6950s are more expensive and require overclocking to reach stock 5870 MHash/sec), but if you want to game the 6950 will offer a more efficient and future proof card (it has better tesselation features, superior crossfire scaling, more VRAM etc.)

WRONG. 6950 1GB can be unlocked to. I come from the forum who really brought this unlocking (techpowerup)

if you are on a single monitor, 6950 1GB may be you best best, as it STOMPS ALL OVER THE 5870 IN GAMING, and is very close in mining. If unlocked to 6970, it even beats it, else it is VERY close to it anyways.

Go for the 6950. 2GB is good too though