The HD 6950 is a slightly more powerful than a GTX 560 Ti, but at the cost of power consumption and heat. The Toxic Edition is a great value since it unlocks to an HD 6970, not to mention the 2GB of VRAM help in high resolutions and demanding games. The GTX 560 Ti you posted is out of stock as well. It's really what you prefer at this point. The GTX 560 Ti is probably a better buy ATM since it is cheaper (the price difference between the HD 6950 and GTX 560 Ti tends not to be justified with performance difference) and it OC like a beast (you can easily get to GTX 570 speeds if you OC correctly). So I would go with the GTX 560 Ti for now, but the HD 6950 if you can get it somehow (the Toxic Edition).
EDIT: 2GB cards are really only useful at this current time if multi-monitor gaming or running a single large monitor @ 2560x1600 reso. If you aren't running a super high resolution monitor then 2GB won't be useful to you for a very, very long time.