Oh, sorry. 5970. Normal 560s in SLI will hit microstuttering issues.
Microstuttering is not a new phenomenon. It will be present in any dual GPU setup, including an HD5970.
@OP: At reference speeds the HD5970 is going to give slightly better performance overall but the two GTX 560s will likely pull ahead in games that take good advantage of DX11 due to Nvidia's superior performance in tessellation. If you overclock then that should put the GTX 560s ahead as well.