Hello is more formal. It is when you see this couple that lives in your society: You don’t want to be too informal because you dont know them very well, and don’t want to be too formal because they aren’t your teachers. Hello is just confusing. It’s when you want to take the middle road or play it safe.
Hi is when you know that person well enough to not be diluded into whether to wait and talk, or just wave and go far away. Hi is when you know for a fact you are going to stand for 2 hours and talk if time permits or even go on a ride.