560TIs are faster than the 6950, quieter, run cooler, and all new nvidia cards overclock incredibly well. 580s have near perfect scaling when in SLI, the second card increases performance by about 95%, so I dont see why a 560 ti wouldnt do the same. Sure, ATI has more memory, but you can always get the 2b 560 ti, and unless you are playing on 3 monitors, the vram shouldnt make a difference, the stock 560 ti vram should be enough to run dual 1080p monitors.
The 580's in SLI don't scale nearly as well as 69xx series in Xfire.
The 560Ti isn't faster, it is about 6% slower then the 6950 on avg
Which is 6 frames out of 100Fps
In this picture, disregard the highlighted as that is the non-ti version.