감사합니다 and 고마워 are kind of expressions when you feel some emotions such as thanks, appreciation and gratitude. 감사합니다 is a formal expression but 고마워 is a informal one.

감사합니다 is more polite expression, and usually 고마워 is used between friends

Both of them are correct. When you have some emtion such as thanks, gratitude and apprecitaion, You can say "감사합니다." or "고마워". You just make sure difference between "감사합니다" and "고마워" "감사합니다" is a formal expression but "고마워" is inforaml. If you want to express your feeling to the ederly, You need to say " 감사합니다." but to friends, you can say "고마워"