There isn't any significant difference. "Hi there" has the idea that you are speaking to someone particular at a distance from you..."Hi, over there! You, across the room, Hi!" Both "Hi" and "High there" are used very often. I don't think of it so much as a greeting, but more of an acknowledgement of someone's presence. If I were to greet someone face to face, I would not say "Hi." I would say "Hello." If I walked into a room, and I saw some friends, I would say "Hi" or "Hi, there" if I was a distance away from them. "Hello" is used both formally and informally. Other greetings that one might use are: "Good morning/afternoon/evening." If it is a first time introduction, one might say, "Good afternoon, nice to meet you." If it is not a first time introduction, one might say "nice to see you" or "nice to see you, again." A lot of situations might necessitate a different type of greeting.

I personally prefer hi there over hi, for some reason i find it friendlier and less formal, you can also go with: hey there, which is obviously very informal.