According to passmark.com:
Radeon 6950 has a score of 3250
The 560ti has a score of 2972
However, this is only a rough reference. Both of these cards will perform exceptionally well. In terms of heat and OC potential, you will have to look at the different suppliers and go off of customer feedback/professional reviews when deciding which one will meet your needs
If it was me personally, I would go with the HIS IceQ X Turbo 2gb version. The reasons being
A) it is only a few dollars more than some other versions.
B) Actually has the best reviews on Newegg
C) It has good eyefinity support if you ever decide to do that
Bonus: There is also the potential to unlock this to a 6970 with the 2gb version... I am not too familiar with that process but feel free to look it up
Basically, this card delivers, and leaves lots of doors of opportunity open to you in the future.
Good luck, and let us know what you decide to go with!