Well, both of them have specific and different meanings (even though they contain the same adjective).
ㄴ/은 것 같다: Is used to mean "I think", "I guess", "It looks like" in Korean.
나는 내일 비가 올 것 같아
(I think it'll rain tomorrow/ I guess it'll rain tomorrow/ It looks like it'll rain tomorrow)
같다: It simply means "to be the same", and you can use it to say that something and another thing are the same.
그 카페테리아는 이 카페테리아와 같아.
( that cafeteria and this cafereria are the same)
I hope I'd helped you! ^^