According to tomshardware the 6950 has awsome scaling . Almost 90%Most of the time. 5870 is not as good in crossfire compared to 6950 . 6950 also has some new features like 3d etc. Therefore i would go with 6950.
The HD 5870 would be a more powerfull solution in a single card setup for gaming at the given resolution but as far a crossfire goes the 6950 all the way!! due to it's fantastic scaling & in the future you might also be working at a higher resolution that could possibly utilise the 2gig frame buffer thus making the 6950 a better all-rounder.