For me I believe that the best way that a business or company can be making the most maximum profit on a product or services is by rendering his product to new customers and clients rather than creating new product for it already existing customers or clients.It depends upon how rate or ratio of your selling. If your sale is too much then both point will be fine for you. But in my point of view selling new product to existing or old customers will be better but if you want to expand then expand your customers. But quality of products should be improve of goods.