First check witch one(sli or crossfire) does your motherboard support. Your best choice would be one 6950. In Crossfire you get better scaling than nvidia with the 6k series from AMD(ati was the name up until the 5k series but that changed).
Also for 1080p resolution 1Gb of RAM is ok, if you plan to go higher you will need more video RAM. Also in time games tend to use more RAM so the 2Gb version of 6950 might pose an advantage, but let's be clear about this, sometimes the 1Gb version is a little faster. Don't know why but that's how it is, but not in all games and not by a lot.
On the TI you chose many posted complaints about the high temperature.
On the 6950 you chose someone posted you can not unlock it, i don't know if that is true but you can only do that on reference design boards, also unlocking might pose some difficulties or might break your card. I would not bother with it.
By the way, 6950 is generally a little faster then gtx560ti.