An instant heating water converter/kettle is an excellent idea if you wish to stay in your bed and stay out of it for a short time to get just one glass of hot water. A water dispenser boils the entire tank and keeps it warm for longer, giving you hot water around the clock. In contrast, the one-cup hot water kettle gives you only one cup at a time & that too when you turn it on. So out of the two, a water dispenser is ideal for people who have more requirements for hot water at a time. The hot water kettles are suitable if you require less quantity at a given time.