Which is better; single pricing or dual pricing?
There is no definitive advantage to either type of pricing. Dual pricing provides an effective mechanism to protect against dilution. When investors buy and sell units, the fund managers have to cancel and create units accordingly. (Sometimes they will buy units from sellers themselves, known as box management). The action of creating and cancelling units to meet supply and demand (inflows and outflows of money) create costs. A dual priced fund provides a mechanism whereby this cost is borne by the buyer or seller that causes the cost. With a single priced unit, this cost is borne by the fund itself, therefore affecting all unit holders of the fund. This is known as dilution.
Please remember that your long-term investment objectives should always take priority over any short-term fluctuations in price.