6950 just a bit faster than 560ti ( deponding on the game a bit ) http://www.guru3d.com/article/radeon-hd-6950-1gb-vs-geforce-gtx-560-ti-review/11 . 550 sli should be just a litlle bit faster but also more expensive i think and all risky stuff like microstutter. Therefore ; 6950 ( 2G ? ) ( and you always have the possibilty to crossfire that later in the future if needed.)
both cards have the same performance depending on the game, some games you get better fps with HD 6950 and some other you get better fps with GTX 560ti, check this
http://www.techpowerup.com/reviews/HIS/Radeon_HD_6950/
i suggest you get the HD 6950 2Gb version it's much more proof than the GTX 560ti some hungry VRAM games like Crysis 2 will benefit from the 2gb, upcoming games, higher resolution 2560× will benefit from it as well
Xbitlabs claims that HD 6950 Toxic Edition is faster than HD 6970 check this out http://www.xbitlabs.com/articles/graphics/display/sapphire-radeon-hd-6950-toxic.html