Who should a lawyer serve: justice or his client?

A defense attorney is ethically obligated to zealously represent their client within the bounds of the law. That does not allow them to use suborned perjury or knowingly misrepresent the law to the court. A prosecutor has an ethical responsibility to insure that justice is served. So if the prosecutor has evidence that the person accused is actually not guilty of the offense charged they are ethically bound to disclose that to the court. The whole system is based on the presumption that each side is represented by equally talented, intelligent and skillful attorneys and that the closest thing to the truth will be revealed through the competition between the two sides.