I think the both are right, and have the same meaning becuse we have structure: be adj to do sth ( be so, too adj to do sth) => I was so silly to believe him be adj enough to do sth => I was silly enough to believe him. (I think, I don't use this sentence in this case, because be adj enough to do something means have a condition or ability to do sth . ex: he is tall enough to play basketball. hope it help you. have a nice day.