For hedonism, pleasure is the supreme ideal, whereas for rationalism, moral law i.e. the categorical imperative is the supreme ideal.
Hedonism centers round the concept of good and evil whereas rationalism centers round the concept of right and wrong.
Hedonism decides the worth of moral action on the consequences or intentions of actions. An action is good if it produces pleasure. Rationalism decides the worth a moral action on the motives behind it. An action is right if it arises from moral law i.e. sense of duty.
Hedonism gives importance to inclinations. It aims at gratification of emotions and impulses. Rationalism gives importance to performance of duty. It aims at Kingdom of Ends i.e. domain of reason.