hi is always used as a greeting but sometimes "hey" is used to grab the attention of a friend/family member.. "hey, have you seen my watch anywhere?"

hey is more casual, usually used between friends

actually, hey can also be used like hi in certain context, but it depends on the contours (tone) or your voice. English is also a tonal language like Chinese, but most people don't realize that.

in my view.hi is more common,choose hey means the relationship appears to be better