I'm currently using a gtx 560ti sc and ive had zero problems with it. I wouldnt mind going with them again except the msi oc numbers are through the roof. Im just not sure if those numbers are out of the box or if you have to do additional ocing.
The MSI card keeps the GPU a bit cooler, which will lead to higher and more consistently sustained boost clocks. The only way the SC would be faster out of the box is if it has a much higher factory overclock. With similar core clocks, the MSI will be faster for the reason listed above.