Not a whole lot
in single card configurations. In dual card configurations the 6950 CF will pull ahead both due to slightly better scaling and it's additional VRAM. For single cards IMO the decision comes down to VRAM vs. Tessellation. I'm a big fan of both but the 560ti can be found with 2Gb of VRAM, the 6950 isn't available with decent tessellation performance. That said the 2Gb 560ti will be more expensive than the 6950 2Gb IIRC. Ideally you'd want to get a card that doesn't compromise on either VRAM, tessellation (AMD doesn't make a card with decent tessellation performance FYI), overall power, and is available at a decent price.