The old customers that has been around and kept faith with the business is suppose to be given a top priority,as this is his actual customers and not the ones that just came in most times in a flash and may never return back patronage.An existing customer is customer that has been patronizing you for long and keep on doing so while new customer is the one that just patronize you newly. In my own view, both of them are so very important in business setting and even for business growth. As for you which one will you prefer to treat specially?