"Lucky you" is used to express that someone, obviously, is lucky. For example: Person 1: I'm going to Paris tomorrow. Person 2: Oh, lucky you! --- "Good for you" is used to congratulate people, generally congratulating them for an achievement. For example: Person 1: I've lost a lot of weight this week! Person 2: Good for you!

"Lucky you" or "Lucky!" can also denote jealousy, envy or even admiration. It is sometimes even a combination. Person 1: "I'm going to Paris tomorrow." Person 2: "Lucky! I wish I was able to go to Paris." (This conveys jealousy.) Person 1: "I'm going to Paris tomorrow." Person 2: "Lucky you! I would love to go to Paris." (This conveys a combination of admiration and excitement for you being able to go to Paris, as well as a little bit of envy.)