which seasonic M12II? that's a whole series. and which cards do you want to SLI?
Seasonic has higher quality components but they still make some rediculous multi 12v rail supplies. if they are the same wattage and both single rail seasonic is "better". Corsairs are also good though, and cheaper.
Never heard of a 700W Seasonic unit.
Obviously you mean a 750W .= which is more the enough power! http://www.seasonicusa.com/M12II-Bronze-650-750-850.htm
Plus the original M12D and more recent M12II series also featured the same power range.
A quality 650W will run 2x560ti's.