My Palit Sonics have been rock solid for the four months that I've had them and a lot of that time has been spent folding 24/7.
So the Sonic is clocked higher and one comes with 2gb of memory. I guess the only benefits to be gained from the extra memory would be with dual monitor setup and resolutions over 1920 x 1200. I've always been somewhat leery of the dual fan setups in sli for some reason. More fans mean better cooling but more db.
Here is something of interest. The Palit Sonic GTX 560 @1ghz in SLI is only 15.5 FPS slower than 2 GTX 580's in SLI in Metro 2033 @1920x1200 but in 1680x1050 there is a very noticeable advantage to the GTX 580 by 27.667. Both test done @ 4 X msaa / 16 X afhttp://www.bjorn3d.com/read.php?cID=2006&pageID=10201