I would be very careful what gtx 560 ti that is bought, if it does not have the mid plate or any meaningful cooling for the power vrm and vram it is not worth it. The 6950 on the other hand are usually come with a mid plate if you buy a reference model which do have the dual bios which are great for the flash if one ever wants to go down that road but not the default choice for owners of this card. The 6950 in the traditional two way crossfire with one or two monitors offer more than enough power even for the most demanding games except for those that have physx enabled.
Reference yes to either option but be very careful what you settle with and not all are created equal. If no power vrm cooling and little or no vram cooling it will not last as long as you may like.