Actually, both are correct. It depends on the person who is talking and it's preferences. For example, the first one sounds better to me, but you can use both. You could use "You seem quiet today", too. This last sentence look more "transitive" to me, because you say that "you look quiet" rather than "being quiet". Using the verb "to be" sounds more permanent and direct to me.
But, anyway, they are all ok! I hope I've helped you!