The 6950 can be compelling because it uses 25-30% less power than the 560 Ti while pushing out the same gaming performance.
However, the GTX 560 Ti will probably work with the video editing stuff better than the 6950 because Nvidia's GPUs have CUDA acceleration which is currently superior to AMD's Radeon 6000 series GPGPU accelerations. AMD makes huge leaps up to Nvidia is this field with their upcoming Radeon 7000 cards but the $200 or so cards might not be out for another month or two.
Basically, the 560 Ti is the better current option for your more professional work while the 6950 would be a better for your gaming. Overall I would have to recommend the GTX 560 Ti over the 6950 because of your non-gaming work.