Hi is more casual It depends on home formal you would like to be

Dear is formal, and Hi is informal. You would only use ‘Hi’ if you know the person and their first name - Hi Mark.

Dear is standard, formal salutation. Hi is informal and you would usually use with a first name, and someone you know 🙂

Dear, Mr. Smith is more formal while Hi Mr. Smith is as if you are talking to a friend.

In formally manner we use mr.Smith to express a formal communication And use dear to communicate in happy way. Both means the same thing.