Fundamental difference is that 優しい is い-adjective while 親切 is な-adjective. 例＝彼は優しい人です。　　彼は親切な人です。 　　　彼は優しいのです。　　彼は親切なのです。 　　　彼は優しくない。　　　　彼は親切ではない。 One more difference is that 優しい can be applied for someone or something while 親切 can be often applied for the action or condition of someone. Refer following examples but just as the theory based on the dictionary. 例＝エコカーは地球に優しい→○　　エコカーは地球に親切→△ 　　　この洗剤は手に優しい→○　　　この洗剤は手に親切→△ Let me say once more, this is the theory based on the dictionary. But 親切 will be used for something when the speaker is mentioning the person who is behind such something as エコカー or 洗剤 etc in his mind.

Good thinking!!!